added new readme in the directory
mention pr folder in whatsnew and CONTRIBUTING
adding a couple of examples

since these are empty files, the eventual tools/ script that will
concatenate these into development.txt can use these as a test case
fix links
apply @takluyver's suggestion
added bolding of "must have"
Merge pull request #4070 from ivanov/doc-pr-folder

New changes should go into pr/ folder