Add Debian and Ubuntu install instructions

Hi
I'm The Silver Searcher package maintainer of Debian.
In Debian unstable/testing and Ubuntu 13.10, package is now available.
I added the installation procedure to README.md.

P.S.
For Ubuntu 13.04, I have prepared the my PPA. (https://launchpad.net/~mizuno-as/+archive/silversearcher-ag)
I also added procedures.

Thanks.