pull request checklist

Description:
  Documentation update
  The developer guide is updated to provide a pull request checklist, a set of prerequisites and checks for new pull requests.
  The developer guide is now linked from the documentation front page

* do new files pass pep8? 
 * no new source files
* do all the tests pass locally (python setup.py test)?
 * yes
* has a new test been provided?
 * no - no functional change
* has iris-test-data been updated?
 * no - not needed
* is the documentation updated to explain the new feature or bug fix?
 * yes
* have the ‘what is new’ and ‘changes’ been updated?
 * no
* does the documentation build without errors?
 * yes
* do the documentation tests pass (make doctest, make extest)?
 * yes
* has a new documentation example been provided?
 * no
* has iris-sample-data been updated?
 * no
* has new generated code been provided using iris-code-generators?
 * no