Add README.md with contributing instructions

README.md

+Alpine Linux aports repository
+==============================
+
+This repository contains the APKBUILD files for each and every
+Alpine Linux package, along with the required patches and scripts,
+if any.
+
+It also contains some extra files and directories related to testing
+(and therefore, building) those packages on GitHub (via Travis).
+
+If you want to contribute, please read the
+[contributor guide](https://wiki.alpinelinux.org/wiki/Alpine_Linux:Contribute)
+and feel free to either submit a git patch on the Alpine aports
+mailing list (<alpine-aports@lists.alpinelinux.org>), or to submit a
+pull request on [GitHub](https://github.com/alpinelinux/aports).