Add volatility dependency installation

We have a task for volatility now (https://github.com/google/turbinia/pull/361), but volatility is not part of the dependencies. Need to look to see what version should be installed (pypi, debian, source, etc) and depending on which version it is, add a reference to it in either the setup.py `extra_requires` or the documentation.

Review the volatility task in pull request 361 and determine which Volatility version and installation source is appropriate. Check setup.py extra_requires and the documentation, then record the dependency in the appropriate location and verify that the installation guidance is complete.

Written by the indexing model from the issue text.