Provide typo fixing guide

[coatless]

Add guide in #79 (comment) to the front matter with a link in the lower left for "spot a typo?"

Potentially, might want to add in hypothes.is (e.g. a "suggest"/annotation layer.)

[daviddalpiaz]

I think I also need to go a step further and create a CONTRIBUTING file in the repo with some additional directions and information.

Not sure how I feel about adding hypothes.is. Do you know something that uses it so I can see it in action?

[coatless]

@daviddalpiaz eh, no one will see the "contributing guide" from the textbook is my worry.

An example with hypothes.is can be found at the R4DS manual:

https://jrnold.github.io/r4ds-exercise-solutions/

[coatless]

@daviddalpiaz we probably should also start thinking about moving some of your repos to an "r-ml" organization on github. This would allow for a shared set of .github resources.

[daviddalpiaz]

I think I need to be thinking about some org structures in general.