Adds a readme to the `/tests/e2e/` folder to provide (a) instructions of how to run the tests locally and (b) links to documentation.
Follow-up to [45570].
Props isabel_brison, desrosj, justinahinon.
Fixes#53550.
git-svn-id: https://develop.svn.wordpress.org/trunk@51904 602fd350-edb4-49c9-b593-d223f7449a82