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
1 file changed
tree: df1d9f7b87d16fb13b2ef06683128d7f06e57743
  1. default.xml
  2. GLOBAL-PREUPLOAD.cfg