Merge branch 'himkp-readme-monaco-languages' into 'master'

Add a README about adding custom languages

See merge request gitlab-org/gitlab!34299

app/assets/javascripts/ide/lib/languages/README.md

+# Web IDE Languages
+
+The Web IDE uses the [Monaco editor](https://microsoft.github.io/monaco-editor/) which uses the [Monarch library](https://microsoft.github.io/monaco-editor/monarch.html) for syntax highlighting. 
+The Web IDE currently supports all langauges defined in the [monaco-languages](https://github.com/microsoft/monaco-languages/tree/master/src) repository. 
+
+## Adding New Languages
+
+While Monaco supports a wide variety of languages, there's always the chance that it's missing something. 
+You'll find a list of [unsupported languages in this epic](https://gitlab.com/groups/gitlab-org/-/epics/1474), which is the right place to add more if needed.
+
+Should you be willing to help us and add support to GitLab for any missing languages, here are the steps to do so:
+
+1. Create a new issue and add it to [this epic](https://gitlab.com/groups/gitlab-org/-/epics/1474), if it doesn't already exist.
+2. Create a new file in this folder called `{languageName}.js`, where `{languageName}` is the name of the language you want to add support for.
+3. Follow the [Monarch documentation](https://microsoft.github.io/monaco-editor/monarch.html) to add a configuration for the new language. 
+    - Example: The [`vue.js`](./vue.js) file in the current directory adds support for Vue.js Syntax Highlighting.
+4. Add tests for the new langauge implementation in `spec/frontend/ide/lib/languages/{langaugeName}.js`.
+    - Example: See [`vue_spec.js`](spec/frontend/ide/lib/languages/vue_spec.js).
+5. Create a [Merge Request](https://docs.gitlab.com/ee/user/project/merge_requests/creating_merge_requests.html) with your newly added language.
+
+Thank you!