manual: add FAQ about .asd files and Good Style

[Tobias C. Rittweiler]

It would probably be good if there was a FAQ explaining that it's good
style trying to keep .asd pure; listing means to achieve that like
IN-ORDER-TO -- pointing to the Unit-Test FAQ as an example; and
giving reasons why it's a good idea (static analysis of .asd files
for automatic test benchs)

[Faré (fahree)]

Not a replacement for a better manual, but I started a "best practices" document:
https://gitlab.common-lisp.net/asdf/asdf/blob/master/doc/best_practices.md

[Robert P. Goldman (rpgoldman)]

I'm reluctant to spawn a lot of outside documents to keep track of. Any objection if I smash this into the manual?

[Faré (fahree)]

No objection. When I started writing it, I had no idea where I was going, and it felt more like bits of a tutorial than bits of a manual. But if you manage to fold it into the manual in a meaningful way, that's great.