From: http://www.mongodb.org/display/DOCS/Choosing+a+Shard+Key
To: http://docs.mongodb.org/manual/core/sharding/#shard-keys

Much of the content is here, http://docs.mongodb.org/manual/core/sharding-internals/#shard-keys

Where exactly the link points to might be something we need feedback on.

Make sure the target is stable.