README: link to docs/developer in Contributing section

The section links to the general osbuild team developer guide, but the
local developer documentation is also very useful.  The page is already
linked from the HACKING.md file, but that could be missed by someone
focusing on the README and following the link to osbuild.org.

README.md

@@ -19,6 +19,8 @@ Images
 
 Please refer to the [developer guide](https://www.osbuild.org/docs/developer-guide/index) to learn about our workflow, code style and more.
 
+See also the [local developer documentation](./docs/developer) for useful information about working with this specific project.
+
 The build-requirements for Fedora and rpm-based distributions are:
 - `gpgme-devel`, `btrfs-progs-devel`, `device-mapper-devel`