Document the new DictionaryContainsKeyValuePairConstraint

[rprouse]

NUnit 3.13 adds a DictionaryContainsKeyValuePairConstraint (issue nunit/nunit#3470, PR nunit/nunit#3486 and another issue requesting nunit/nunit#3747)

Syntax is like

var dictionary = new Dictionary<string, string> { { "Hello", "World" }, { "Hola", "Mundo" } };
Assert.That(dictionary, Does.ContainKey("Hola").WithValue("Mundo"));