Added README in boost include directory [skip ci].

MANIFEST.in

 include LICENSE README.rst bootstrap.py buildout.cfg requirements.txt
 recursive-include doc conf.py *.rst
 recursive-include bob *.cpp *.h *.hpp
+include bob/core/include/bob.core/boost/README.txt

bob/core/include/bob.core/boost/README.txt

+This directory contains BOOST header files that were copied from BOOST version 1.56, which are required to provide consistent and correct output of the random number distributions provided by bob.core.random.
+These header files are modified such that they work with our package; particularly, the namespace of the BOOST functions are changed bob::core::random, i.e., to assure that they overwrite the ones from BOOST version < 1.56.
+
+The BOOST license applies. You can download the license from http://www.boost.org/LICENSE_1_0.txt.
+
+If you have BOOST version 1.56 or greater installed, you can safely ignore/delete this directory.