Add imagemagick as project dependency

[lym]

When I ran the tests the first time, I got this error:

This was an obvious indication that I needed to install the imagemagick library. I think adding this as a project dependency will be helpful for folks setting up the project for the first time.

[tarzan]

Imagemagick indeed is a an implicit dependency for converting (avatar) images.

Feel free to add it to the Readme :)

[lym]

I will create a small PR for this.

[fatboypunk]

done in #490