Add basic emoji development docs

changelogs/unreleased/update-emoji-digests-with-latest-from-gemojione.yml

+---
+title: 'Update emojis. Add :gay_pride_flag: and :speech_left:. Remove extraneous comma
+  in :cartwheel_tone4:'
+merge_request:
+author:
+type: changed

doc/development/README.md

@@ -22,6 +22,7 @@ comments: false
 
 - [UX guide](ux_guide/index.md) for building GitLab with existing CSS styles and elements
 - [Frontend guidelines](fe_guide/index.md)
+- [Emoji guide](fe_guide/emojis.md)
 
 ## Backend guides
 

doc/development/fe_guide/emojis.md

+# Emojis
+
+GitLab supports native unicode emojis and fallsback to image-based emojis selectively
+when your platform does not support it.
+
+# How to update Emojis
+
+ 1. Update the `gemojione` gem
+ 1. Update `fixtures/emojis/index.json` from [Gemojione](https://github.com/jonathanwiesel/gemojione/blob/master/config/index.json).
+    In the future, we could grab the file directly from the gem.
+    We should probably make a PR on the Gemojione project to get access to
+    all emojis after being parsed or just a raw path to the `json` file itself.
+ 1. Ensure [`emoji-unicode-version`](https://www.npmjs.com/package/emoji-unicode-version)
+    is up to date with the latest version.
+ 1. Run `bundle exec rake gemojione:aliases`
+ 1. Run `bundle exec rake gemojione:digests`
+ 1. Run `bundle exec rake gemojione:sprite`
+ 1. Ensure new sprite sheets generated for 1x and 2x
+    - `app/assets/images/emoji.png`
+    - `app/assets/images/emoji@2x.png`
+ 1. Ensure you see new individual images copied into `app/assets/images/emoji/`
+ 1. Ensure you can see the new emojis and their aliases in the GFM Autocomplete
+ 1. Ensure you can see the new emojis and their aliases in the award emoji menu
+ 1. You might need to add new emoji unicode support checks and rules for platforms
+    that do not support a certain emoji and we need to fallback to an image.
+    See `app/assets/javascripts/emoji/support/is_emoji_unicode_supported.js`
+    and `app/assets/javascripts/emoji/support/unicode_support_map.js`