RFR: Added section for newcomers [v4]

[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.

Jesper Wilhelmsson has updated the pull request incrementally with one additional commit since the last revision:

  why is my change rejected?

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Changes:
  - all: https://git.openjdk.java.net/guide/pull/38/files
  - new: https://git.openjdk.java.net/guide/pull/38/files/3d23ad7e..8f1bfb4a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=guide&pr=38&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=guide&pr=38&range=02-03

  Stats: 33 lines in 1 file changed: 30 ins; 0 del; 3 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