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

.gitignore

@@ -33,3 +33,6 @@
 
 # User-accessible hook for adhoc debugging scripts
 /support/debug.php
+
+# Users can link binaries here
+/support/bin/*

src/applications/config/option/PhabricatorCoreConfigOptions.php

@@ -84,7 +84,8 @@ public function getOptions() {
             "of the user running Phabricator (normally 'apache', 'httpd', or ".
             "'nobody'). Here you can add extra directories to the \$PATH ".
             "environment variable, for when these binaries are in ".
-            "non-standard locations."))
+            "non-standard locations. Note that you can also put binaries in ".
+            "`phabricator/support/bin`."))
         ->addExample('/usr/local/bin', pht('Add One Path'))
         ->addExample("/usr/bin\n/usr/local/bin", pht('Add Multiple Paths')),
        $this->newOption('tokenizer.ondemand', 'bool', false)

support/bin/README

@@ -0,0 +1,3 @@
+Phabricator will look in this directory for binaries (like "git", "hg", "svn",
+"diff", etc) before looking elsewhere. If you want Phabricator to use a
+specific version of some binary, copy or symlink it here.