README.md link to main site repo

Make it easy to find https://activitywatch.net/ repo from this one

Same idea as ActivityWatch/activitywatch.github.io#41

Alternatively, could look more like:

> The [activitywatch.net](https://activitywatch.net/) repository can be found at [ActivityWatch/activitywatch.github.io](https://github.com/ActivityWatch/activitywatch.github.io).

Author: mrienstra

README.md

@@ -6,8 +6,10 @@ ActivityWatch Documentation
 
 Documentation for [ActivityWatch](https://activitywatch.net), the free and open-source automated time tracker.
 
-The docs are hosted at [docs.activitywatch.net](https://docs.activitywatch.net/) (by [Read the Docs](https://readthedocs.org/)).
+Hosted by [Read the Docs](https://readthedocs.org/).
 
+https://activitywatch.net/ ➝ [ActivityWatch/activitywatch.github.io](https://github.com/ActivityWatch/activitywatch.github.io)  
+https://docs.activitywatch.net/ ➝ this repo
 
 # Build