Explicitly use python version number

It is needed for systems where python points to python3

Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Use Markdown in README

Switch to Markdown syntax in README. I did the minimal amount of changes
to make the rendering on Github look sane. We can prettify it even more
later.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>