RFR: Section about the release process

[Jesper Wilhelmsson]

Answering some common questions around the release process.

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Commit messages:
 - Milestone cheat sheet
 - Section on release process

Changes: https://git.openjdk.java.net/guide/pull/62/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=62&range=00
  Stats: 75 lines in 2 files changed: 71 ins; 3 del; 1 mod
  Patch: https://git.openjdk.java.net/guide/pull/62.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/62/head:pull/62

PR: https://git.openjdk.java.net/guide/pull/62