CONTRIBUTING.md: Fix link to unit tests

[jonasdn]

Fixes: #755

[krichardsson]

Looking good, but maybe also update the link to "building and flashing instructions" to the same style

There are actually 3 different ways of linking:

1. to our web - https://www.bitcraze.io/documentation/repository/crazyflie-firmware/master/
2. to github - https://github.com/bitcraze/crazyflie-firmware/tree/master/docs
3. to local docs dir - /docs/

For markdown files in the root (usually readme.md and contributing.md) I think 1. is the best option and 2. the worst

[jonasdn]

Looking good, but maybe also update the link to "building and flashing instructions" to the same style

There are actually 3 different ways of linking:

1. to our web - https://www.bitcraze.io/documentation/repository/crazyflie-firmware/master/
2. to github - https://github.com/bitcraze/crazyflie-firmware/tree/master/docs
3. to local docs dir - /docs/

For markdown files in the root (usually readme.md and contributing.md) I think 1. is the best option and 2. the worst

Thanks for review! I've updated building and flashing in the README.md as well.