readme: make contribution section more obvious

Move link to contribution guidelines to it's own section in README.

Fixes: 8c64bda5a0 ("doc: add contribution guidelines including code style")
Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>

README.md

@@ -102,6 +102,7 @@ This will install templates and other config files to `/custom/location/share/ae
 and man pages to `/custom/location/share/man`. This extra location will have lower
 priority than the XDG locations but higher than the fixed paths.
 
+## Contributing
 
 Anyone can contribute to aerc. Please refer to [the contribution
 guidelines](https://git.sr.ht/~rjarry/aerc/tree/master/item/CONTRIBUTING.md)