Note packages needed to download https pages with SNI.

In order for python-requests to be able to download from
https sites using SNI (such as PopIt), it needs some
extra python packages

* pyOpenSSL
* ndg-httpsclient
* pyasn1

which require libffi-dev and libssl-dev for compilation.
fixup! Note packages needed to download https pages with SNI.

Add more details to the readme file.