assessment/README.md: Start listing references for assessment background

For me (and others with no formal background in formal assessments),
it would be good to have a list of suggested reading focused on
bringing people up to speed with existing research.  I'm seeding this
list with the freely available content that Greg pointed out:

On Fri, Mar 22, 2013 at 07:29:07AM -0400, Greg Wilson wrote:
> Doodling on concept inventories is kind of like doodling on grand
> unified field theory --- there's a _lot_ of background knowledge
> required, and as someone who's in the early stages of climbing that
> particular hill, it's a pretty big one.  As a start, have you worked
> through "How Learning Works" and/or any of the literature (both pro and
> con) on concept inventories in other fields?  Allison Tew's thesis from
> Georgia Tech, and pp 17-139 of Juha Sorva's thesis
> (http://lib.tkk.fi/Diss/2012/isbn9789526046266/isbn9789526046266.pdf),
> are also useful & relevant.
Merge pull request #29 from wking/assessment-reading

assessment/README.md: Start listing references for assessment background