Document related mapmatcher project

[nik9000]

Adds a reference to the mapmatcher project to the related projects documentation. mapmatcher is a smallibrary of matchers for Map and List concentrating on readable error messages that report the entire structure at once.

[nhojpatrick]

Going to try and kick start hamcrest, so if you want to get it merged, please rebase from the branch v2.3-candidates. Still trying to understand how has permissions to perform a release.