doxygen now recognizes README and .txt files

doxygen now recognizes README and .txt files

The default doxygen configuration file only accepts source files such
as php or python.  Some of our documentation is in flat files which we
might want to include in our Doxygen inline documentation. The culprit
is that those document files needs to respect Doxygen syntax, i.e. the
text content need to be enclosed in a comment block such as:

 /*!
 \page page_title My Page Title

 My awesome documentation
 */

Doyxgen 1.8 supports markdown syntax, we might want to switch to that
eventually.

Change-Id: I78a0ce36314b2a7b9f89395e3ae3902b17cf5038