Describe the new endpoint option in the readme

Signed-off-by: Tj Holowaychuk <tj@vision-media.ca>

Readme.md

@@ -25,6 +25,10 @@ The following examples demonstrate some capabilities of knox and the s3 REST API
       , bucket: 'learnboost'
     });
 
+By default knox will send all requests to the global endpoint (bucket.s3.amazonaws.com).
+This works regardless of the region where the bucket is. But if you want to manually set
+the endpoint (for performance reasons) you can do it with the `endpoint` option.
+
 ### PUT
 
 Below we do several things, first we read _Readme.md_ into memory,