How to add unsupported language.

README.md

@@ -15,6 +15,57 @@ Syntax highlighting for Dart and Flutter, which supports lots of languages and t
 - [All available languages](https://github.com/pd4d10/highlight/tree/master/highlight/lib/languages)
 - [All available themes](https://github.com/pd4d10/highlight/blob/master/flutter_highlight/lib/themes)
 
+## How to add unsupported language
+
+1. Install nodejs and gulp.  
+Highlight.dart uses [nodejs](https://nodejs.org/en/) and [gulp](https://gulpjs.com/) from command line to generate language syntax definition files, so please install nodejs and gulp-cli.
+
+2. make sure your language is [supported by highlight.js](https://github.com/highlightjs/highlight.js/blob/main/SUPPORTED_LANGUAGES.md).
+3. add your supported language repository as git submodule under `vendor` folder  
+  **Note:** if the `main` key's value of `pacakge.json` in your language repository does not exists, please correct it(and then create a PR to the repository) or the syntax definition file won't be generated.
+4. edit `tool/highlight.js` function allModes`:
+for exmaple, given I'm trying to add support for GDScript:
+```javascript
+export function allModes(){
+...
+  const items = [
+    ...dirs.map(file => ({
+      name: path.basename(file, path.extname(file)),
+      factory: require(path.resolve(dir, file)),
+      community: false
+    })),
+    {
+      name: "vue",
+      factory: require("../vendor/highlightjs-vue/vue").definer,
+      community: true
+    },
+    {
+      name: "graphql",
+      factory: require("../vendor/highlightjs-graphql").definer,
+      community: true
+    },
+    {
+      name: "gn",
+      factory: require("../vendor/highlightjs-GN").definer,
+      community: true
+    },
+    {
+      name: "solidity",
+      factory: require("../vendor/highlightjs-solidity").definer,
+      community: true
+    }, // <- don't forget the comma!
+    /* Below are appended lines*/
+    {
+      name: "gdscript",
+      factory: require("../vendor/highlightjs-gdscript").definer,
+      community: true
+    }
+    /* append end */
+  ];
+  ...
+```
+5. change your working directory to `tool` and run gulp in command line, and then use `dart format [your directory]` command to format generated dart files.
+
 ## License
 
 MIT