Add contribution readme for technical steps

CONTRIBUTION.md

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+# Technical documentation for contributors
+
+## Brand vs app
+The name of the brand, is defined as a subfolder.
+To clarify things, we have 2 platforms for our apps: iOS and macOS.
+Therefore each brand will end up with 2 apps (one for iOS and one for macOS).
+
+## At the beginning we have the following:
+- custom brand folders, each with an info.json file, and xcassets
+- we have the kiwix/apple code (as a seperate repository). 
+
+    We do not add any swift code to custom apps. All swift code related to custom apps is already included in kiwix/apple repository, it is just "not used" in the Kiwix apps.
+
+The `kiwix/apple` repo is checked out into `/apple` folder, while the custom app repo (this one), is checked out under `/custom` folder.
+
+Since most of the code and files are already in the `/apple` folder, we copy the `/custom` folder under it, so it ends up in: `/apple/custom`. We do the rest of the building from that folder.
+
+## What we need in order to build a custom app?
+
+### A Zim file
+This needs to be downloaded from the url given in the `info.json`, and placed under the branded folder, eg: `/custom/dwds`
+
+### A .plist file
+The plist file is a list of settings, that is used by xcode for the given target to be built. It is in a required xml format.
+
+It is created under the folder with the same name, eg.: `dwds/dwds.plist`
+
+It should contains the following common variables:
+```
+APP_STORE_ID = ${APP_STORE_ID}
+CUSTOM_ABOUT_TEXT = ${CUSTOM_ABOUT_TEXT}
+CUSTOM_ABOUT_WEBSITE = ${CUSTOM_ABOUT_WEBSITE}
+CUSTOM_ZIM_FILE = ${CUSTOM_ZIM_FILE}
+SETTINGS_DEFAULT_EXTERNAL_LINK_TO = ${SETTINGS_DEFAULT_EXTERNAL_LINK_TO}
+```
+
+as a matter of fact these key/values are going to be the same for each brand. The final value for these are defined in the relevant .xcconfig files.
+
+The plist file should also contain the following variables that have their value inlined (with example values here):
+```
+CUSTOM_ABOUT_WEBSITE = "https://www.dwds.de"
+SETTINGS_SHOW_EXTERNAL_LINK_OPTION = false
+SETTINGS_SHOW_SEARCH_SNIPPET = false
+```
+
+Since the plist files are pretty much the same,
+and we want to reflect any changes made in the main (kiwix/apple) repo as well, creating them is done in the following steps:
+
+1) we make a copy of the `Support/Info.plist` file contained in the main repo (kiwix/apple) -> `Custom.plist`
+2) we add all the common variables to it. This is done by using the macOS built in commandline tool named `usr/libexec/PlistBuddy`. Note that  the plist file expects not only the keys and values, but the type of the variable as well (which we do not have in our custom `info.json` file). Currently we use string and boolean types.
+3) we make a copy of this `Custom.plist` into the custom folder eg: -> `dwds/dwds.plist`
+4) we add the custom values for this brand, using the same tool `PlistBuddy` mentioned above
+
+This way we end up with a plist file dedicated to a specific brand containing all the variables and values it needs.
+
+### An xcconfig file
+Under the custom app folder we create a branded 
+xcconfig file, eg: `dwds/dwds.xcconfig`, it is containing brand specific (build time) values, it should have the following (with example values):
+```
+CUSTOM_ABOUT_TEXT = Für Schreibende, Lernende, Lehrende und Sprachinteressierte:...
+APP_STORE_ID = id6473090365
+SETTINGS_DEFAULT_EXTERNAL_LINK_TO = alwaysLoad
+SETTINGS_SHOW_EXTERNAL_LINK_OPTION = False
+SETTINGS_SHOW_SEARCH_SNIPPET = False
+CUSTOM_ZIM_FILE = dwds_de_dictionary_nopic_2023-12-15
+```
+
+## Enforced language?
+If we use enforcing a language for a brand, what that really means is: we cannot use any of the localization folders contained in the main (kiwix/apple) repo, instead we need to make a copy of a single language folder, eg: `de.lproj` and place it into the custom folder eg: `/custom/dwds/de.lproj` and include only that in the project. 
+
+(Note: only including / excluding does not work as the references to those language folders in the final Xcode project file will mess up. So we need a copy of the folder, and only this way it works as expected.)
+
+An extra step is required here, we also need to set the DEVELOPMENT_LANGUAGE to this value (eg. "de").
+
+# A new custom_project.yml file
+The main `kiwix/apple` repo contains a `project.yml` file, it describes all the common build and project settings, and all the templates we re-use to create a final xcode project file, which is used to build the custom apps. At the end each custom app will be a separate target we can build.
+Therefore we need dynamically create the `custom_project.yml` file, which will import the `project.yml`, and sets up the new targets (one for each brand), it looks more or less, like this:
+```
+include:
+- project.yml
+targets:
+  dwds:
+    templates:
+    - ApplicationTemplate
+    configFiles:
+      Debug: custom/dwds/dwds.xcconfig
+      Release: custom/dwds/dwds.xcconfig
+    settings:
+      base:
+        DEVELOPMENT_LANGUAGE: de
+        INFOPLIST_FILE: custom/dwds/dwds.plist
+        INFOPLIST_KEY_CFBundleDisplayName: DWDS
+        INFOPLIST_KEY_UILaunchStoryboardName: SplashScreen.storyboard
+        MARKETING_VERSION: 2023.12.3
+        PRODUCT_BUNDLE_IDENTIFIER: org.kiwix.custom.dwds
+    sources:
+    - path: custom/dwds
+    - path: custom/SplashScreen.storyboard
+      destinationFilters:
+      - iOS
+    - path: Support
+      excludes:
+      - '*.xcassets'
+      - Info.plist
+      - '**/*.lproj'
+```
+Once this is ready, we can call `xcodegen` on it:
+```xcodegen -s custom_project.yml``` which will create the `Kiwix.xcodeproj`, and that will contain the original `Kiwix` target and all the branded targets as well eg. `dwds`.
+
+## A splash screen
+The splash screen file is going to be the same for each custom app (it is using a branded logo image from the branded xcassets). As you can see from the above example, this file's final location is going to be: `/apple/custom/SplashScreen.storyboard`.
+