Explain that matplotlib must be built before the HTML documentation

Attempting to generate the HTML documentation right after cloning the
repository results in a cryptic error message as one of the required
files, mpl-data/matplotlibrc, is created when matplotlib is built. It
seems that other people have come across this problem in the past [1],
so mentioning it here might be of help to newcomers.

[1] http://sourceforge.net/mailarchive/message.php?msg_id=21478615
Show custom error message if we don't have a matplotlibrc file

Even if the README.txt file explicitly mentions that matplotlib must
have been built before the documentation can be generated, a better
error message will be of help to those developers who happen not to
read it.
Show custom error message if we don't have a matplotlib installation
Mention that matplotlib must be installed before building the docs
Merge pull request #1660 from vterron/master

Explain that matplotlib must be built before the HTML documentation