RFR: Section on backporting

[Jesper Wilhelmsson]

Guidance on backporting.
Also moved "Backing out a change" to the testing section where I believe it fits better. This section is unchanged.

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Commit messages:
 - Section on backporting

Changes: https://git.openjdk.java.net/guide/pull/66/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=66&range=00
  Stats: 225 lines in 1 file changed: 133 ins; 92 del; 0 mod
  Patch: https://git.openjdk.java.net/guide/pull/66.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/66/head:pull/66

PR: https://git.openjdk.java.net/guide/pull/66