Add more docs about filter-files option

[micahjsmith]

The skip-related options (skip, extend-skip, skip-glob, extend-skip-glob, skip-gitignore) all interact with filter-files but don't mention it in their own documentation, and filter-files from its name wouldn't come up in text searches for "skip". Adds a mention of filter-files usage to the skip-related options docs.

[timothycrosley]

Thanks for these improvements 🎉