document lt, gt, lte, gte

this documents the new range encoding, as discussed in https://github.com/rvagg/node-levelup/issues/201
maybe the simplest way to describe it would be all at once:

`gt` (greater than) `lt` (less than) `gte` (greater than or equal) `lte` (less than or equal), describe a range for the stream to retrive, for example `{lte: 'zzzzzz', gte: 'a'}` will stream all the key/value pairs where the `key <= 'a' && key <= 'zzzzzz'` (`lt` and `gt` are similar, but are used to express exclusive ranges)