RFR: Section on PRs

[Jesper Wilhelmsson]

Some tips for creating pull requests.

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Commit messages:
 - Section on PRs

Changes: https://git.openjdk.java.net/guide/pull/79/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=79&range=00
  Stats: 42 lines in 1 file changed: 42 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/guide/pull/79.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/79/head:pull/79

PR: https://git.openjdk.java.net/guide/pull/79