RFR: Section on backporting [v3]

[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.

Jesper Wilhelmsson has updated the pull request incrementally with two additional commits since the last revision:

 - Update src/index.md
   
   Co-authored-by: Magnus Ihse Bursie <mag at icus.se>
 - Update src/index.md
   
   Co-authored-by: Magnus Ihse Bursie <mag at icus.se>

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

Changes:
  - all: https://git.openjdk.java.net/guide/pull/66/files
  - new: https://git.openjdk.java.net/guide/pull/66/files/d5f7a7fc..a9f359bb

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=guide&pr=66&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=guide&pr=66&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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