Move developer note to non-rendered section.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
Solutions for 1.2-3, expand 1.1 solution.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
Merge pull request #302 from ezyang/exercises

Solutions for 1.2-3, expand 1.1 solution.