Hygiene: README for browser tests

* Make a note that the diff page uses the "Diff test" page
* Ensure the wikitext for the Barack Obama article comes under the
information about BArack Obama rather than being cut by the note on
ConfirmEdit extension

Change-Id: I2eb2a488991586bd600739118c67cd27c925440d
Merge "Hygiene: README for browser tests"