add reference to capa install instructions to PR suggestions

[mike-hunhoff]

On another note. I noticed when trying to do a pull request, it gave a few tips, such as running lint.py from the scripts section The issue was, I only cloned the capa-rules repo, so it didn't have that scripts folder in it, as that's in the capa repo

This can be confusing to new users who went straight to writing capa rules without checking out the capa docs first.