Add initial ACCESSIBILITY.md file (re: #7713)

ACCESSIBILITY.md

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+# Accessibility & Compatibility
+
+iD aims to make mapping as easy possible for as many people as possible. To this end,
+we recognize that everyone has different backgrounds, abilities, and technologies,
+and therefore different needs. Developing for the "average user" will inevitably
+fail to serve some proportion of mappers. Broadly speaking, iD should strive to
+follow [universal design](https://en.wikipedia.org/wiki/Universal_design) principles.
+
+This is a living document that details the usability of iD across a number of dimensions,
+with the intent of identifying and addressing problem areas.
+
+Symbols used in this document:
+
+- ✅ Full support
+- 🟩 Full support assumed but unverified
+- 🟠 Partial support
+- ❌ No appreciable support
+
+## Cross-Browser Compatibility
+
+As a web app, iD's browser support is fundamental. The user experience should be
+as equivalent as possible across the latest versions of all modern browsers on all
+major operating systems. When possible, functionality unsupported by older browsers
+should fallback gracefully without breaking other aspects of the app.
+
+This table covers high-level compatibility, with individual features to be detailed
+elsewhere in this document.
+
+|   | Browser | Notes |
+|---|---|---|
+| ✅ | Chrome | |
+| ✅* | Firefox | *Minor known issues ([#7132]) |
+| ✅ | Safari | |
+| 🟩 | Opera | Infrequently tested |
+| 🟩 | Edge | Infrequently tested |
+| 🟠 | Internet Explorer | Infrequently tested. IE has been discontinued, but IE 11 is still maintained. iD polyfills ES6 features on IE 11, with varying success. |
+| 🟠 | Mobile browsers | iD has not yet been fully optimized for mobile devices, but some editing is usually possible. |
+
+[#7132]: https://github.com/openstreetmap/iD/issues/7132
+
+--
+
+_Further sections coming soon…_