Add requirements for building the user manual

[tleedjarv]

Closes #545 (although still nothing about LANG/LC_TYPE plan)

[gdt]

We don't have CONTRIBUTING.md now. I just would like it to be clear that you need these for strings regen, even if you don't care about the manual.

[gdt]

But I guess it is clear enough, and this is way better than before, so calling it good enough.