Update CONTRIBUTING.md
Update CONTRIBUTING.md

The Android and Linux testing bullets should be sub-bullets under
the first. They require 2 space indent on github.

Change-Id: I30a8d5ea93631ce13cd9f788fab6113a0fd0fc2e
Signed-off-by: David Kilroy <159cd46f289d7961b38b0171b53c05be4aaef0b6@arm.com>

ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build