README: Add a short intro to OSv

Add on the top of README.md a short introduction to what OSv is.
If someone gets to our github page, https://github.com/cloudius-systems/osv,
and scrolls down, it's strange that we only explain how to build OSv,
without first mentioning what it is.

Fixes #148

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>