add description for tox tasks
add description for tox tasks

For example:
default environments:
py27     -> run test suite for the application with python2.7
py36     -> run test suite for the application with python3.6
lint     -> run linter (prospector) to ensure the source code
corresponds to our coding standards
docs     -> build readthedocs documentation

additional environments:
coverage -> run test suite with code coverage for the application with
/usr/bin/python
eslint   -> run the JavaScript linter (requires gulp installed)

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