Add some extra depenencies to documentation

[maartenuni]

The readme states doesn't state that one needs to install libldap2-dev as an dependency. If you don't pip won't be able to install some packages that are in requirements.txt. Also it might be helpfull to inspringing developers to name those dependencies prior to pip. Otherwise, you'll run into cryptic error messages running

pip install -r requirements.txt

Edit the list of requirements is quite gentle when it says:

• When using mariadb: client and dev libraries

However, pip requires it when running the pip command above, perhaps its better if we just see it as an dependency.

[tymees]

Oops :P

Never thought of that, as my PC has those dependencies installed already

[maartenuni]

No problem:-). I would sometimes be nice when pip just says: "The wheel bladibla needs libbladibla" instead of showing lots of red text :)