Direct the user to python-ldap installation docs in the README

As demonstrated by the 230 upvotes and 180k+ views on
https://stackoverflow.com/questions/4768446/i-cant-install-python-ldap
- it doesn't usually install the first time.

As the README didn't mention the prerequisites, I immediately turned to
Google after encountering an error, which was less helpful than the
official docs.

(I moved the Python compatibility info to another section, as it looked
out of place after I added the link to python-ldap installation
instructions.)