Add badges instead of labels name in the contribution guidelines

[smoia]

Following a comment from @tsalo in #315 , it's much much better to put badges for labels instead of their names in the contribution guideline.

Possible Implementation

• Look at the labels page in GitHub (e.g. here)
• Use this website to see the colours of the labels
• Go to shields.io
• Create a badge for each label with the right colours and a flat style (the closest to the new GitHub label style)
• Add them to the documentation

[merelvdthiel]

Hey, would it be ok if i work on this issue?