docs: add gitlab - github repo sync documentation to readme

README.md

@@ -48,6 +48,9 @@ Strategies and tactics to achieve objectives:
 - [Installation and Configuration](#installation-and-configuration)
   - [Local Environment](#local-environment)
   - [GitLab Project](#gitlab-project)
+    - [GitLab - GitHub Synchronization](#gitlab---github-synchronization)
+    - [GitLab CI Settings](#gitlab-ci-settings)
+    - [GitLab CI Nightly Pipeline](#gitlab-ci-nightly-pipeline)
 - [Usage](#usage)
   - [Usage Examples](#usage-examples)
 - [Contributing](#contributing)
@@ -162,8 +165,32 @@ tools/setup-repo
 . tools/secrets.sh
 ```
 
+Run `tools/update-repo` from time to time to update repository dependencies.
+
 ### GitLab Project
 
+#### GitLab - GitHub Synchronization
+
+To create working GitLab to GitHub repository synchronization:
+
+- Prepare GitHub token, let's call it `GitLab GitHub Sync`, with scopes:
+  - `repo` (and `repo:status`, `repo_deployment`, `public_repo`, `repo:invite`, `security_events`)
+  - `workflow`
+  - `write:packages` (and `read:packages`)
+  - `delete:packages`
+- Have or create GitHub repository
+- Set up GitLab GitHub synchronization: Settings
+  - Repository
+    - Mirroring repositories, *Expand*
+      - *Add new mirror*:
+        - Git repository URL: **<https://user@github.com/org/repo.git>**, please replace _user_, _org_, and _repo_
+        - Mirror direction: **Push**
+        - Password: **`GitLab GitHub Sync` token**
+        - Keep divergent refs: **On** or **Off**
+        - Mirror only protected branches: **On** (all release and pre-release branches should be set as protected, otherwise GitHub release would fail on non-existent branch) or **Off**
+
+#### GitLab CI Settings
+
 Set up release and GitLab CI Linter tokens as the GitLab group or the GitLab project variable:
 
 - **GL_TOKEN**: [GitLab Personal Access Token](https://gitlab.com/-/profile/personal_access_tokens) with scopes `api` and `write_repository`. Shouldn't be protected otherwise GitLab CI job `lint` fails with an error `Server said HTTP Error 401: Unauthorized: https://gitlab.com/api/v4/ci/lint`.
@@ -178,15 +205,15 @@ Set up release and GitLab CI Linter tokens as the GitLab group or the GitLab pro
         - Flags:
           - Protect variable: **On** (GitLab & GitHub Releases and GitLab CI Linter will work only on protected branches) or **Off** (insecure - accessible to anybody, who can create a commit in GitLab)
 
+#### GitLab CI Nightly Pipeline
+
 Set up the GitLab scheduled pipeline:
 
 - CI/CD
   - Schedules
     - *New schedule*
       - *Fill* and *Save pipeline schedule*
 
-Run `tools/update-repo` from time to time to update repository dependencies.
-
 ## Usage
 
 Simply fork the repository at [GitLab](https://gitlab.com/xebis/repository-template/-/forks/new) or [GitHub](https://github.com/xebis/repository-template/fork), **delete** all git tags, and **tag** the last commit to a desired starting version, e.g. `v0.0.0`. Clone the repository with `--recursive` option, run `tools/setup-repo`, and enjoy!