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 <johannes.schindelin@gmx.de>
Helped-by: Derrick Stolee <derrickstolee@github.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>