Include "unknown sources" in Android instructions

Add instructions on how to allow running APKs from unknown sources.

BUG=none
TEST=manual

Change-Id: I61539cff081555d9255454fa3f30b986c2c5c6f6
Reviewed-on: https://chromium-review.googlesource.com/1154543
Reviewed-by: Ted Choc <[email protected]>
Commit-Queue: Thiemo Nagel <[email protected]>
Cr-Commit-Position: refs/heads/master@{#579345}
1 file changed