Add technical description of ARM-11: Execution Model

I have not yet written the supplementary sections on testing/evaluation,
alternatives, etc, but I wanted to publish this more widely now that the
technical description is substantively complete so it could get wider exposure
and commentary.

Closes #26.
Fix erroneous mix-up of the link title vs description
Fix numbering in README. GRR.
Add new sections on Content Service and Report Service.
Merge pull request #27 from ahpook/master

Added final service sections to ARM-11 technical writeup.
Merge branch 'master' of ssh://git@github.com/puppetlabs/armatures.git