start a global config w/documentation

This doesn't turn on anything (yet), just includes some guidelines for
people who are thinking about changing/enabling things.
Bug: 29606287
Test: `repo upload` still works
Change-Id: I397f53aed417fe9bfd1a00dac2d0d5b380f66e10

GLOBAL-PREUPLOAD.cfg

@@ -0,0 +1,3 @@
+# Settings in here control defaults for `repo upload` checks for all projects
+# listed in the manifest.  Do not turn on things here without first consulting
+# the wider Android team, and make sure you send out a PSA before landing.