document lt, gt, lte, gte

this documents the new range encoding, as discussed in https://github.com/rvagg/node-levelup/issues/201
I'd make this less short but moar easy:

`'gt'` (greater than), `'gte'` (greater than or equal): ...

the stream output will only include __k/v pairs whose keys__ are gre....

I'd move `(can be either...)` out of the parentheses.

If not provided the output will __start at__ the lowest key in the database.

I'd remove the parentheses at:  See also (`'lt', 'lte'`, and `'range'`).

The last sentence starts with a lower case letter