Use README.md as doxygen main page

From doxygen documentation:
http://www.stack.nl/~dimitri/doxygen/manual/config.html#cfg_use_mdfile_as_mainpage

USE_MDFILE_AS_MAINPAGE

If the USE_MDFILE_AS_MAINPAGE tag refers to the name of a markdown file
that is part of the input, its contents will be placed on the main page
(index.html). This can be useful if you have a project on for instance
GitHub and want to reuse the introduction page also for the doxygen
output.

Bug: 73530
Change-Id: I812a32f0bd11f05aa10a73db334bccd49875adca
Merge "Use README.md as doxygen main page"