RFR: Clearly state that users should create branch

[David Holmes]

On Fri, 13 Jan 2023 13:49:11 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> 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)

Seems quite reasonable.

Thanks.

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Marked as reviewed by dholmes (Author).

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