add errata markdown draft

This is only a partial draft.  Wanted to see what you all think of the format before I add more text.

Also, there is already an errata.txt in this directory.  Is this errata based on the current version of the book?  Should we just add on to this text file, or port relevant entries over to the markdown format?

The sources I have been using for the markdown errata list:
* the book forum
* a list I compiled when I went through the book
* things students have pointed out to me in class
* things other instructors have mentioned in campfire