Instruct devs to invoke from project root using `dev/fakes3.sh`

* This is a little easier to type than `./fakes3.sh` at least for me anyways.
* In case they don't do it from there I've already added it to the .gitignore in 20c3e9f20c3e9f9b21ca442f84bedbbdc86cbf4b65aa20f

Applies to #217
Merge pull request #220 from Martii/CONTRIBUTING

Instruct devs to invoke from project root using `dev/fakes3.sh`