Start improving CONTRIBUTING.md, as suggested in issue #8037:
1. Incorporate the few lines we had in coding-style.md into CONTRIBUTING.md.
This was mostly a pointer to Seastar's coding style anyway, so it's not
helpful to have a separate file which hopeful developers will not find
anyway.
2. Mention the Scylla developers mailing list, not just the Scylla users
mailing list. The Scylla developers mailing list is where all the action
happens, and it's very odd not to mention it.
3. The decisions that github pull requests are forbidden was retracted
a long time ago, so change the explanation on pull requests.
4. Some smaller phrasing changes.
Refs #8037.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20210214152752.1071313-1-nyh@scylladb.com>
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