tests/README

doc: make links for hg role

The system has a bit of smartness:

  :hg:`update`     -> links to hg.1.html#update
  :hg:`help diffs` -> links to hg.1.html#diffs

To run the tests, do:

cd tests/
python run-tests.py

See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.