Added README.

README.md

+# Dynare reference manual
+
+Source for the Dynare's reference manual which is available at
+<http://www.dynare.org/manual-unstable>. The documentation is obtained
+using [Sphinx](http://www.sphinx-doc.org/) Pyhthon Documentation
+Generator. To build the `html` version of the reference manual just type:
+
+```bash
+~$ make html
+```
+
+on the command line. The reference manual will be available under
+`src/build/html`, provided all the following (debian package)
+requirements are met:
+
+ - python3,
+ - python3-sphinx,
+ - python3-recommonmark, and
+ - libjs-mathjax