Guidelines for history extraction from bc

@gvwilson, @wking @r-gaia-cs, continuing discussion here.

It would be good to have a central reference (blog post?) for the
process of lesson history extraction. The current instructions for
this are a bit diffuse.

I think there needs to be clarification on the following (as I type, no order):

1) Where progress is tracked (currently labeled issues on bc)

2) Where new PRs on lesson material should be made.

3) Can these PRs be merged or are they pending conversion to the new
lesson template? If merging then how to manage overview on personal
repos?

4) Should we do this entirely in personal repos or make repos in swcarpentry?

5) Is bc frozen?


Additionally:

6) If (3) is true should an extra step in the extraction be to put
everything in a pages/ directory, following the new lesson template?
Otherwise we have to filter-branch again when merging PRs.
What I'm suggesting:

- extract in personal repo
- audit this repo
- if pass audit, move to swcarpentry repo as-is
- *now* freeze bc and make new issues on new swcarpentry repo

This means

1. we don't have to deal with the whole issue stack before moving to swcarpentry repo. 
2. greater visibility

Also, with the files now in the top level of the lesson template we might not even need to rebase to merge a pull-request in.