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

Author: Víctor Terrón

doc/README.txt

@@ -39,6 +39,11 @@ for the initial run (which builds the example gallery) to be done,
 then run "python make.py html" again. The top file of the results will
 be ./build/html/index.html
 
+Note that ../lib/matplotlib/mpl-data/matplotlibrc' is not version
+controlled, but created when matplotlib is built. This means that the
+HTML documentation cannot be generated immediately after checking out
+the source code: you will have to run ``python setup.py build`` first.
+
 To build a smaller version of the documentation (without
 high-resolution PNGs and PDF examples), type "python make.py --small
 html".