Docs: add "Developing a backend"

docs/development/developing-backend.md

@@ -0,0 +1,76 @@
+# Developing a backend
+
+Soapbox expects backends to implement the [Mastodon API](https://docs.joinmastodon.org/methods/).
+
+At the very least:
+
+- [instance](https://docs.joinmastodon.org/methods/instance/)
+- [apps](https://docs.joinmastodon.org/methods/apps/)
+- [oauth](https://docs.joinmastodon.org/methods/apps/oauth/)
+- [accounts](https://docs.joinmastodon.org/methods/accounts/)
+- [statuses](https://docs.joinmastodon.org/methods/statuses/)
+
+Soapbox uses feature-detection on the instance to determine which features to show.
+By default, a minimal featureset is used.
+
+## Feature detection
+
+First thing, Soapbox fetches GET `/api/v1/instance` to identify the backend.
+The instance should respond with a `version` string:
+
+```js
+{
+  "title": "Soapbox",
+  "short_description": "hello world!",
+  // ...
+  "version": "2.7.2 (compatible; Pleroma 2.4.52+soapbox)"
+}
+```
+
+The version string should match this format:
+
+```
+COMPAT_VERSION (compatible; BACKEND_NAME VERSION)
+```
+
+The Regex used to parse it:
+
+```js
+/^([\w+.]*)(?: \(compatible; ([\w]*) (.*)\))?$/
+```
+
+- `COMPAT_VERSION` - The highest Mastodon API version this backend is compatible with. If you're not sure, use a lower version like `2.7.2`. It MUST follow [semver](https://semver.org/).
+- `BACKEND_NAME` - Human-readable name of the backend. No spaces!
+- `VERSION` - The actual version of the backend. It MUST follow [semver](https://semver.org/).
+
+Typically checks are done against `BACKEND_NAME` and `VERSION`.
+
+The version string is similar in purpose to a [User-Agent](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent) string.
+The format was first invented by Pleroma, but is now widely used, including by Pixelfed, Mitra, and Soapbox BE.
+
+See [`features.ts`](https://gitlab.com/soapbox-pub/soapbox-fe/-/blob/develop/app/soapbox/utils/features.ts) for the complete list of features.
+
+## Forks of other software
+
+If your software is a fork of another software, the version string should indicate that.
+Otherwise, Soapbox will use the minimal featureset.
+
+### Forks of Mastodon
+
+Mastodon forks do not need the compat section, and can simply append `+[NAME]` to the version string (eg Glitch Social):
+
+```
+3.2.0+glitch
+```
+
+### Forks of Pleroma
+
+For Pleroma forks, the fork name should be in the compat section (eg Soapbox BE):
+
+```
+2.7.2 (compatible; Pleroma 2.4.52+soapbox)
+```
+
+## Adding support for a new backend
+
+If the backend conforms to the above format, please modify [`features.ts`](https://gitlab.com/soapbox-pub/soapbox-fe/-/blob/develop/app/soapbox/utils/features.ts) and submit a merge request to enable features for your backend!