Add README and initial start on style guide text.

Refs #9521

Code/Mantid/docs/DocumentationStyleGuide.md

+Documentation Style Guide
+=========================
+
+This guide describes the formatting that should be followed when documenting Mantid functionality. The documentation is built using [Sphinx](http://sphinx.pocoo.org/) and when using Sphinx most of what you type is reStructuredText. For more information on reStructeredText see:
+
+* https://pythonhosted.org/an_example_pypi_project/sphinx.html
+* http://docutils.sourceforge.net/rst.html
+* http://docutils.sourceforge.net/docs/user/rst/quickref.html
+* http://docutils.sourceforge.net/docs/user/rst/cheatsheet.txt
+

Code/Mantid/docs/README.md

+The Mantid documentation is written in [reStructuredText](http://docutils.sourceforge.net/rst.html) and processed using [Sphinx](http://sphinx.pocoo.org/). To install Sphinx type
+
+    easy_install -U Sphinx
+
+CMake produces a `doc-html` target that is used to build the documentation. The output files will appear in a `html` sub directory of the main `build/docs` directory.
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