Edited Issue: Record external contributions [3071]

As noted here, we want to start recording external contributions:
http://blog.nuget.org/20130211/workflow-changes-for-codeplex-issues.html

Either in an acknowledgements.txt file, or in the existing changelog.txt, we need to record pull requests that have been accepted, tracking the following:

1. The contributor's username
2. The title of the work item fixed
3. The work item number

We should include bug fixes and features alike. It would be great if it's possible to collect this information for pull requests already accepted into 2.3 too.