[libcxx] Add BOT_OWNERS.txt

[DavidSpickett]

Summary

Buildkite doesn't provide a way to list bot owners so currently
we are pinging people on Discord and Phabricator.

Which works ok until that person is on vacation. This file gives us
a place to list multiple people, or group contacts for each bot.

I've stuck to the CODE_OWNERS.txt format because there's no great
reason to change it.

[DavidSpickett]

Though being able to add discord/phab names would be nice and sod's law that the format took D and P already. We can figure something out if that would be useful.

[ldionne]

@daltenty Heads up to add the AIX information to this file.

Thanks @DavidSpickett !

[DavidSpickett]

I will land this then, others can add to it.

Also, I saw another CODE_OWNERS using "H" for Phabricator Handle so that is an option.

[DavidSpickett]

Note H for Phabricator handle.

libcxx/docs/AddingNewCIJobs.rst

 config appropriately.
 
 We try to keep the pipeline definition file as simple as possible, and to
 keep any script used for CI inside ``libcxx/utils/ci``. This ensures that
 it's possible to reproduce CI issues locally with ease, understanding of
 course that some setups may require access to special hardware that is not
 available.
 
+Finally, add your contact info to ``libcxx/utils/ci/BOT_OWNERS.txt``. This will
+be used to contact you when there are issues with the bot.
+
 Testing Your New Job
 ====================
 
 Testing your new job is easy -- once your agent is set up (if any), just open
 a code review and the libc++ CI pipeline will run, including any changes you
 might have made to the pipeline definition itself.
 
 Service Level Agreement

libcxx/utils/ci/BOT_OWNERS.txt

+This file is a list of the people or groups responsible for ensuring that bots
+are running correctly. They will also deal with platform specific issues that
+cannot be figured out by the community alone.
+
+The list is sorted by surname and formatted to allow easy grepping and
+beautification by scripts. The fields are: name (N), email (E), web-address
+(W), PGP key ID and fingerprint (P), description (D), snail-mail address
+(S), Phabricator handle (H) and IRC handle (I). Each entry should contain at
+least the (N), (E) and (D) fields.
+
+N: Linaro Toolchain Working Group
+E: [e-mail]
+D: Armv7, Armv8, AArch64