Add documentation on accessibility. (#113)

* Add documentation on accessibility.

the-draupnir-project/planning#81.

Author: Gnuxie

docs/contributing/accessibility.md

@@ -0,0 +1,31 @@
+# Accessibility
+
+## Render methods for `org.matrix.custom.html` (DeadDocument)
+
+To render command responses to Matrix, we use a custom JSX factory from
+[interface-manager](https://github.com/the-draupnir-project/interface-manager)
+to provide us with elements from `org.matrix.custom.html` (and the DOM that
+facilitates this is called `DeadDocument`).
+
+The general rule to keeping message content accessible is use semantic HTML
+elements over arbitrary ways of maintaining structure.
+
+The following describes what we should be doing to improve the accessibility of
+message content. Pull request reviewers should check rendered messages match the
+recommendations in this document.
+
+### Headings
+
+Headings should be used at the beginning of each section within a message. This
+is because users of screen readers who depend on keyboard navigation need
+headings to properly skip over sections.
+
+### Descriptive text for indicators
+
+If emoji or symbols are used to show a status, then the meaning should be
+written explicitly. For example, the `!draupnir protections` command shows
+whether each command is enabled or disabled with the green circle `🟢` and red
+circle emoji `🔴`. But next to each we also say what these mean with `(enabled)`
+or `(disabled)`. This is important not only for users struggling with colour
+perception, but also because it can be unclear to users what the emoji means in
+the first place.