The run_cross_tests.sh script was renamed to oslo_run_cross_tests
in a recent change. Update the documentation to reflect that.
Change-Id: Id16ea9e77e8771758d23c7f39dc3e664df4728e6
The bulk of the documentation was coming from README.rst, probably
too much, so I moved it to it's own file under docs.
Change-Id: Iec30feae9c152af7d8b6a1b79670b3f0149d2360