Listing os libs on readme

Found out that only this one where required:

python-dev,  libmemcached-dev and zlib1g-dev (for memcached), libpq-dev (for Postgres)
This are the ones on Ubuntu (I used a Ubuntu:14.04 docker image to check this, since it's a very common OS).
Anyway, if someone don't use this OS they can also check for their equivalents.
this should fix #95