Add Alpine Linux installation instructions (#191)

The first distribution to have nheko in it's official repositories! 🎉

    Right now nheko is only in the `testing` repository, but this will
    be moved to `community` after a while (after which it's available
    for installation out of the box).

README.md

@@ -42,6 +42,14 @@ sudo layman -a matrix
 sudo emerge -a nheko
 ```
 
+#### Alpine Linux (and postmarketOS)
+
+Make sure you have the testing repositories from `edge` enabled. Note that this is not needed on postmarketOS.
+
+```sh
+sudo apk add nheko
+```
+
 ### Build Requirements
 
 - Qt5 (5.7 or greater). Qt 5.7 adds support for color font rendering with