Merge branch 'ps/doc-recommend-b4' into jch

Project-specific configuration for b4 has been introduced, and the
documentation has been updated to recommend using it as a
streamlined method for submitting patches.

* ps/doc-recommend-b4:
  b4: introduce configuration for the Git project
  MyFirstContribution: recommend the use of b4
  MyFirstContribution: recommend shallow threading of cover letters

diff --cc Documentation/SubmittingPatches
Simple merge