Add README.chromium file.
Add README.chromium file.

So that we can add chromite to the Chrome checkout under
src/third_party.

BUG=None
TEST=None

Change-Id: I3345a7fddf8aa69e42558953520912a6b7ec54d2
Reviewed-on: https://gerrit.chromium.org/gerrit/45937
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite