Improve docs for the merchandise site

Thom Wiggers
2018-02-13T11:49:59+01:00

See #570

### Short description
* Improves the docs for the merchandise package
* Adds pylint to dev requirements (such the new config doesn't break people running it themselves)

### Pylint output:

```
Using config file /home/thom/git/thalia/concrexit/website/.pylintrc

--------------------------------------------------------------------
Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)

```