Add note about mercurial
Add note about mercurial

Some tests need mercurial to pass.

Closes #3320

safwanrahman/readthedocs.org,safwanrahman/readthedocs.org,davidfischer/readthedocs.org,davidfischer/readthedocs.org,rtfd/readthedocs.org,safwanrahman/readthedocs.org,safwanrahman/readthedocs.org,rtfd/readthedocs.org,rtfd/readthedocs.org,rtfd/readthedocs.org,davidfischer/readthedocs.org,davidfischer/readthedocs.org