wiki: schema missing for NUnit 3.0

[ghost]

https://github.com/nunit/docs/wiki/XML-Formats

Would be nice if you could add a schema for NUnit 3.0 too, as you did for NUnit 2.0

[CharliePoole]

Schema is more than just a documentation issue. NUnit V2 actually used a schema to validate it's XML output.

If we do this, we should do the same, otherwise it's only a promise we may not be keeping.

[mikkelbu]

We have an issue about creating a schema (and I've considered doing it, but so far have not had time for it). See nunit/nunit-console#13

[jnm2]

Framework PR is up: https://github.com/nunit/nunit/pull/2929