Add documentation for installing on Debian/Ubuntu and CentOS/RHEL

[smacz42]

Addresses #36 and #35

However, when you said in #36 (comment):

Also, the ldapcherry dependency are already packaged by CentOS (with Epel) and Debian/Ubuntu, I should probably mention it and document it, it will be less fragile than pip.

Did you want to have the packages in the requirements*.yml files installed with the OS's package management rather than having pip do the install when running setup.py?

[coveralls]

Coverage remained the same at 80.757% when pulling b9743ef on smacz42:andrewcz-ldapcherry-issue36 into 856157a on kakwa:master.