README: add short explanation of this repository

Inspired by the Alpine aports README: https://gitlab.alpinelinux.org/alpine/aports/-/blob/master/README.md?ref_type=heads

README.md

-# postmarketOS aports
+# postmarketOS aports repository
+
+This repository contains the APKBUILD files for postmarketOS-specific packages, along with the required patches and scripts, if any.
+
+There are many more packages defined in the [Alpine Linux aports](https://gitlab.alpinelinux.org/alpine/aports/)] on which these packages depend.
 
 Helpful resources:
 * [How to create a package](https://wiki.postmarketos.org/wiki/Create_a_package)