Modify README.rst

This patch fixes README.rst written in Markdown even though it's in the
ReStructuredText format.

This patch also adds some instructions for newcomers to make it clear
how to get started.

Change-Id: I8bf909aac691e90ae9b3b2a0d9341f3f33e2e39d
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>