document new 'db' option.

README.md

@@ -133,6 +133,8 @@ The `location` argument is available as a read-only property on the returned Lev
 
 * `'keyEncoding'` and `'valueEncoding'` *(string, default: `'utf8'`)*: use instead of `encoding` to specify the exact encoding of both the keys and the values in this database.
 
+* `'db'` *(object, default: LevelDOWN)*: LevelUP is backed by [LevelDOWN](https://github.com/rvagg/node-leveldown/) to provide an interface to LevelDB. You can completely replace the use of LevelDOWN by providing a "factory" function that will return a LevelDOWN API compatible object given a `location` argument. For further information, see [MemDOWN](https://github.com/rvagg/node-mem/), a fully LevelDOWN API compatible replacement that uses a memory store rather than LevelDB. Also see [Abstract LevelDOWN](http://github.com/rvagg/node-abstract-leveldown), a partial implementation of the LevelDOWN API that can be used as a base prototype for a LevelDOWN substitute.
+
 Additionally, each of the main interface methods accept an optional options object that can be used to override `encoding` (or `keyEncoding` & `valueEncoding`).
 
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