Documentation: add ReviewingGuidelines

Documentation: add ReviewingGuidelines

Add a reviewing guidelines document including advice and common terminology
used in Git mailing list reviews. The document is included in the
'TECH_DOCS' list in order to include it in Git's published documentation.

Helped-by: Johannes Schindelin <redacted>
Helped-by: Derrick Stolee <redacted>
Helped-by: Junio C Hamano <redacted>
Helped-by: Josh Steadmon <redacted>
Signed-off-by: Victoria Dye <redacted>
Signed-off-by: Junio C Hamano <redacted>