Convert README and INSTALL to markdown.

Convert README and INSTALL to markdown.

The grutatxt project is dead, so we need to switch to something else
eventually. Fortunately, only little changes are required for the
switch to the markdown markup language as there are only two files in
the source tree which are formated with grutatxt. This patch converts
both files and adjusts the Makefile to run the markdown converter
instead of the grutatxt script.