add tox links to README

Author: stavxyz

README.md

@@ -1,4 +1,5 @@
 # tox-pyenv
 [![latest](https://img.shields.io/pypi/v/tox-pyenv.svg)](https://pypi.python.org/pypi/tox-pyenv)
 [![Circle CI](https://circleci.com/gh/samstav/tox-pyenv/tree/master.svg?style=shield)](https://circleci.com/gh/samstav/tox-pyenv/tree/master)  
-tox plugin that uses `pyenv which` to find python executables
+
+[tox](https://tox.readthedocs.org/en/latest/) plugin that uses `pyenv which` to [find python executables](https://testrun.org/tox/latest/plugins.html#tox.hookspecs.tox_get_python_executable)