Add Alpine installation instructions

This is copypasted from the Mirage installation instructions.

docs/INSTALL.md

@@ -88,6 +88,18 @@ commands:
     $ cp -r ~/.config/mirage/* xyz.mx_moment.moment/config/moment
     $ cp -r ~/.local/share/mirage/* xyz.mx_moment.moment/data/moment
 
+#### Alpine Linux / postmarketOS
+
+If you are on the Edge channel of Alpine Linux or postmarketOS, Moment can be
+installed right from the testing repositry:
+
+```sh
+apk add moment
+```
+
+If you are unsure about what Edge is and want to read more about it, you can do
+so on the [Alpine Wiki](https://wiki.alpinelinux.org/wiki/Edge).
+
 ## Manual Installation
 
 **Qt 5.12+**, **Python 3.6+** (with **pip** to install packages from the