apk: define arch=source qualifier

This is similar to what we agreed on for Debian in #57.

Author: gernot-h

PURL-TYPES.rst

@@ -53,7 +53,8 @@ apk
 - The ``name`` is the package name. It is not case sensitive and must be
   lowercased.
 - The ``version`` is a package version as expected by apk.
-- The ``arch`` is the qualifiers key for a package architecture.
+- The ``arch`` is the qualifiers key for a package architecture. The special value
+  ``arch=source`` identifies an Alpine source package as built by ``abuild srcpkg``.
 - Examples::
 
       pkg:apk/alpine/[email protected]?arch=x86