Add note about filter option

Fixes #57.

Author: glasser

readme.md

@@ -66,6 +66,8 @@ decompress('unicorn.zip', 'dist', {
 });
 ```
 
+*Note that in the current implementation, **`filter` is only applied after fully reading all files from the archive in memory**. Do not rely on this option to limit the amount of memory used by `decompress` to the size of the files included by `filter`. `decompress` will read the entire compressed file into memory regardless.*
+
 ##### map
 
 Type: `Function`