Location sharing: don't hardcode API key

In an effort to make it easier for forks to (a) use their own
API keys (b) change map styles or maybe even providers, move
the MapTiler key out of the source code and pass it in via
env var or property.

Also refactor the utility classes slightly to keep all the URL
related functions together, to reduce the chance of collisions
when maintaining such forks.

docs/maps.md

@@ -0,0 +1,42 @@
+# Use of maps
+
+<!--- TOC -->
+
+* [Overview](#overview)
+* [Local development with MapTiler](#local-development-with-maptiler)
+* [Making releasable builds with MapTiler](#making-releasable-builds-with-maptiler)
+* [Using other map sources or MapTiler styles](#using-other-map-sources-or-maptiler-styles)
+
+<!--- END -->
+
+## Overview
+
+Element Android uses [MapTiler](https://www.maptiler.com/) to provide map
+imagery where required. MapTiler requires an API key, which we bake in to
+the app at release time.
+
+## Local development with MapTiler
+
+If you're developing the application and want maps to render properly you can
+sign up for the [MapTiler free tier](https://www.maptiler.com/cloud/pricing/).
+
+Place your API key in `local.properties` with the key
+`services.maptiler.apikey`, e.g.:
+
+```properties
+services.maptiler.apikey=abCd3fGhijK1mN0pQr5t
+```
+
+## Making releasable builds with MapTiler
+
+To insert the MapTiler API key when building an APK, set the
+`ELEMENT_ANDROID_MAPTILER_API_KEY` environment variable in your build
+environment. 
+
+## Using other map sources or MapTiler styles
+
+If you wish to use an alternative map provider, or custom MapTiler styles,
+you can customise the functions in
+`features/location/api/src/main/kotlin/io/element/android/features/location/api/internal/MapUrls.kt`. 
+We've kept this file small and self contained to minimise the chances of merge
+collisions in forks.