Rename HACKING.md to CONTRIBUTING.md

GitLab picks up on this filename to show some extra UI, so let's just
use it instead of HACKING.

README.md

@@ -24,7 +24,8 @@ Please try to add reproducible steps and the relevant version of libfolks.
 
 If you want to contribute functionality or bug fixes, please open a Merge
 Request (MR). For more info on how to do this, see GitLab's [help pages on
-MR's]. Please also follow our coding conventions, as described in HACKING.md
+MR's]. Please also follow our coding conventions, as described in
+CONTRIBUTIONS.md
 
 If libfolks is not translated in your language or you believe that the current
 translation has errors, you can join one of the various translation teams in

tests/README

-See the "Debugging tests" section in HACKING.