RFR: New text about mailing lists

[Jesper Wilhelmsson]

The last three types of lists, used for expert groups, could use a better explanation. I'm not sure how these types of
lists are used though.

-------------

Commit messages:
 - New text about mailing lists

Changes: https://git.openjdk.java.net/guide/pull/17/files
 Webrev: https://webrevs.openjdk.java.net/guide/17/webrev.00
  Stats: 60 lines in 2 files changed: 49 ins; 7 del; 4 mod
  Patch: https://git.openjdk.java.net/guide/pull/17.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/17/head:pull/17

PR: https://git.openjdk.java.net/guide/pull/17