add initial alpine packaging

notes:
- to build the package, you must use the cargo version from the edge branch
  (by building on edge or by installing it manually)
- building from git requires some work (abuild supports snapshots for getting
  the release from git, but the version number would remain unchanged)
- the apkbuild doesn't include any packaging tests (as i don't know what to include)

alpine/README.md

@@ -0,0 +1,7 @@
+# building
+
+1. [set up your build
+   environment](https://wiki.alpinelinux.org/wiki/Include:Setup_your_system_and_account_for_building_packages)
+   
+2. run `abuild` (or `abuild -K` if you want to keep the source directory to make
+   rebuilding faster)