RFR: Clearly state that users should create branch

Fri Jan 13 13:55:57 UTC 2023

A recent PR submission to the JDK made me aware that the guide does not strongly enough emphasize the importance of creating a separate branch. 

(If you do put your changes directly in `master`, the Skara bots will put up harsh warnings, which is not a nice way to welcome someone who has tried to do everything right, including reading the guide)

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Commit messages:
 - Clearly state that users should create branch

Changes: https://git.openjdk.org/guide/pull/94/files
 Webrev: https://webrevs.openjdk.org/?repo=guide&pr=94&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/guide/pull/94.diff
  Fetch: git fetch https://git.openjdk.org/guide pull/94/head:pull/94

PR: https://git.openjdk.org/guide/pull/94