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.
Merge pull request #1 from bennylope/pylibmc-docs

Specify requirement that pylibmc be in requirements.txt