Reframe proposal guideline as a "how to contribute" document

[mjskay]

Once we finalize the guideline process, I think we'll want to reframe the guideline wiki doc as a more gentle "how to contribute" doc that is accessible to outsiders, e.g. by starting with the "how to make minor changes" block first and telling people how to make a pull request.

Then perhaps the "edit" link in the bookdown doc could link to that wiki page.

TODOs:

• reframe as "how to contribute"
• restructure to not have topic branches but use a "dev" branch
• reference this doc in the README
• reference this doc in the guidelines (e.g. in intro)
• have the edit but in the guidelines go to this doc by changing the following line in the index.Rmd:

  edit: https://github.com/transparentstats/guidelines/edit/master/%s
  

[chatchavan]

For reference: once this wiki page is done, change the following line in the index.Rmd

edit: https://github.com/transparentstats/guidelines/edit/master/%s