doc/design: proposal for 'r' branches

Summary:
Introduce a proposal for adopting a new branch naming scheme, review it
publicly so that current users can discuss together.  Add it to source
control to serve as documentation.

Test Plan:
Ran through the 'examples' section to verify it's an
improvement.

Reviewers: jdevenpo, phab

Reviewed By: phab

CC: arcyd, jwilkin1, anightin, mmallon, spacific, cbailey, pwainwri, pwach, oparanal, dstockl2, sjervis, pmurfitt, tislam, apearce16, whoy1, dharesig, nustund1, jhall1, eyu2, atoussai, ckhazan, jfrullo, lhaynes6, druoso, slee601, ksaunder, sthura-i, jprice30, aalmonte, cbond, ldarbiny

Differential Revision: https://all.phab.dev.bloomberg.com/D37427
testbed/arcyon: make test less flaky

Ordering of tasks seems to be a little flaky when they're created within
a short time of each-other.  Work around this by querying only the first
task created during the exercises.

Test Plan:
$ ./testbed/arcyon/exercise_arcyon.sh
Arcyon task-query: fix ordering

It looks like the '--order' option wasn't really finished, fix a crash
and add a new 'arcyon/acceptance_tests.sh' to exercise various options.

Fix the '--status' help.

Test Plan:
$ ./testbed/arcyon/acceptance_tests.sh