Fix #40. Added vivado's `cmake` troubleshooting

[andreaskurth]

Regarding #40, I prefer #42. I think the Vivado problem is too specific to warrant a FAQ entry. What do you think?

[alessandrocapotondi]

For me we could merge both the PRs since the Vivado environment can be consider as a specific use-case. But if you are against this solution we can keep you one.

[andreaskurth]

Alright, let's keep both.

I took the liberty to add another commit to this branch to format the document, fix some typos, and remove whitespace errors

[alessandrocapotondi]

@accuminium, thanks!