selinux: Document how to quickly iterate policy changes

Knowing what to change in the policy is often not quite obvious, and
takes several attempts. Show how to do this much more quickly than
repeated rpm/image builds.

selinux/HACKING.md

@@ -0,0 +1,11 @@
+# Changing cockpit's SELinux policy
+
+The clean way is to edit the policy files and then rebuild the rpms and image with
+`test/image-prepare -q fedora-XX`.
+
+To iterate more quickly, locally `./configure` the build tree with
+`--enable-selinux-policy=targeted`, then you can quickly recompile and install
+the policy into the `c` SSH target with:
+```sh
+make cockpit.pp && scp cockpit.pp c:/tmp/ && ssh c semodule -i /tmp/cockpit.pp
+```