Correct a typo: s/at time/at times/

Implement some suggestions from Karl:
* Improve the sentence comparing to Rserve
* Add more context about the structure of the protocol buffer schema.
  Section 2 is very high level / abstract, then section 3 dives into the
  R interface without ever describing field types, tag numbers, etc.
  Added three sentences or so to the beginning of section 3 before
  diving into the R examples.



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rprotobuf@773 edb9625f-4e0d-4859-8d74-9fd3b1da38cb