Add docs around file watching rules.

[jennifer-shehane]

Users ask about file watching and how the rules work quite frequently and There is not documentation to point them to.

[jennifer-shehane]

Related: #97

[richdouglasevans]

Can I work on this one @jennifer-shehane ? It's assigned to @chrisbreiding who might have written a sweet guide already thus my asking instead of diving right in.

[jennifer-shehane]

@richdouglasevans Oh, sure. @chrisbreiding originally wrote most of the code for file watching, but I'm sure he and I would be happen to review any contribution you make. 😄

[richdouglasevans]

In light of #829 being merged can this ticket be Closed? Steer me if there's more work to be done and I'll get it sorted.

[jennifer-shehane]

👍