README.md: Improve contribution section
README.md: Improve contribution section

The markdown syntax in the contribution section is incorrect, which is
why the links appear on the same line.

Improve the contribution section by making it more explicit what the
links are about.

Message-Id: <d63157fa3fac94bb344344fd3ccc60a57892beff@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla