Add troubleshooting docs

[apeltzer]

We should try looking at open issues and potential solving points (e.g. missing mksquashfs, missing pipeline configuration, singularity bind paths) and add these to the troubleshooting sections as these questions are asked quite frequently either on slack or the github issue trackers. Golden rule:

"Have a FAQ entry for it, whenever something has been asked for >= 3 times!"

[apeltzer]

e.g #103

[ewels]

Done: https://nf-co.re/usage/troubleshooting