Ignore and README for support/bin

Summary:
See D5561. Ref T2378.

• Add support/bin/* to .gitignore so any symlinks or binaries won't get picked up by Git.
• Add a README so Git preserves the directory and there's at least some documentation of its existence.

Test Plan: ummmmm

Reviewers: jevripio, codeblock, btrahan

Reviewed By: jevripio

CC: aran

Maniphest Tasks: T2378

Differential Revision: https://secure.phabricator.com/D5562