Specify requirement that pylibmc be in requirements.txt

Instructing that a requirement should be added to a project's `requirements.txt` file is not, on its face, taken to mean that a requirement must be added to that file specifically. pip allows for including files so it suffices that a requirement be added to an included `requirements file`. In this case however the `pylibmc` requirement must be explicitly added to the `requirements.txt` file. That should be stated explicitly with explanation.

This should solve a problem for at least [a few other people](https://encrypted.google.com/search?hl=en&q=heroku%20pylibmc%20errors) too.