[MERGE] repository docs (cleaner still) and GraphIndex

[Robert Collins]

This merge adds the repository docs writeup, with the index stuff I had
in it cleaned up and the generic parts factored out to a new developer
text file. 

I've also added a GraphIndex - a graph aware index which is 20% smaller
than our knit indices today (at least for me on my repositories).

I plan to work on hooking this up to knits now, but I think the
interface is sufficient for the time being, and I think its pretty clean
and amenable to layering on top of/parallel implementations.

These things are quite closely related - the index api pressure comes
from repositories needs, which is why I put them in the same branch.

-Rob

-- 
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