RFR: Added section for newcomers

[Jesper Wilhelmsson]

Added a section to the introduction to set the expectations for new developers in the OpenJDK.

I also moved the mail list section up since it's also relevant in this context and should (imho) be the first thing to learn about when thinking about joining the community.

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Commit messages:
 - Added section for newcomers

Changes: https://git.openjdk.java.net/guide/pull/38/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=38&range=00
  Stats: 118 lines in 1 file changed: 72 ins; 46 del; 0 mod
  Patch: https://git.openjdk.java.net/guide/pull/38.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/38/head:pull/38

PR: https://git.openjdk.java.net/guide/pull/38