Add doc for NewPullRequest

[kevinburkesegment]

I recently mixed up "Head" and "Base" and the error message was not clear. Add some guidance to help people avoid making the same mistake.

This is copied word for word from the existing Github docs.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.92%. Comparing base (2b8c7fa) to head (f6299f1).
Report is 82 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #3208 +/- ##
==========================================
- Coverage 97.72% 92.92% -4.80% 
==========================================
Files 153 171 +18 Lines 13390 11582 -1808 ==========================================
- Hits 13085 10763 -2322 - Misses 215 726 +511 - Partials 90 93 +3 

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[gmlewis]

Thank you, @kevinburkesegment - would you like to sign the CLA so that I can merge this, or would you prefer I open up a separate PR and copy the original text from the official GitHub documentation into that one instead?

[kevinburkesegment]

I believe I signed the CLA but if you're still not seeing it on your end please feel free to duplicate the work. I do not need credit, I'd just like it to be fixed so I don't make the same mistake 6 months from now. Thanks

[gmlewis]

I believe I signed the CLA but if you're still not seeing it on your end please feel free to duplicate the work. I do not need credit, I'd just like it to be fixed so I don't make the same mistake 6 months from now. Thanks

Excellent, thank you, @kevinburkesegment !
LGTM.
Merging.