Developer documentation

The lint rules don't currently exist, but would apply to `mxcUrlToHttp` and `getHttpUriForMxc`

docs/media-handling.md

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+# Media handling
+
+Surely media should be as easy as just putting a URL into an `img` and calling it good, right?
+Not quite. Matrix uses something called a Matrix Content URI (better known as MXC URI) to identify
+content, which is then converted to a regular HTTPS URL on the homeserver. However, sometimes that
+URL can change depending on deployment considerations.
+
+The react-sdk features a [customisation endpoint](https://github.com/vector-im/element-web/blob/develop/docs/customisations.md)
+for media handling where all conversions from MXC URI to HTTPS URL happen. This is to ensure that
+those obscure deployments can route all their media to the right place.
+
+For development, there are currently two functions available: `mediaFromMxc` and `mediaFromContent`.
+The `mediaFromMxc` function should be self-explanatory. `mediaFromContent` takes an event content as
+a parameter and will automatically parse out the source media and thumbnail. Both functions return
+a `Media` object with a number of options on it, such as getting various common HTTPS URLs for the
+media.
+
+**It is extremely important that all media calls are put through this customisation endpoint.** So
+much so it's a lint rule to avoid accidental use of the wrong functions.