Document how a large suite can be partitioned into individually targetable buckets #778

The framework added an option to partition a large suite into numbered buckets. This was done in nunit/nunit#4391

The console/engine side was then added in nunit/nunit-console#1345

Ability to partition by fixture was added in nunit/nunit#5165

Documentation should be as a "how to" style of document within https://github.com/nunit/docs/tree/master/docs/articles/nunit/writing-tests, in a style similar to https://github.com/nunit/docs/blob/master/docs/articles/nunit/writing-tests/dependent-tests.md