Add tip for slashes in the prefix

Adriaan · web-flow · commit 43b4a0ec1059 · 2016-08-23T13:17:06.000+01:00
diff --git a/README.md b/README.md
@@ -103,7 +103,7 @@ The ACL to apply to the objects.
 
 ### prefix
 
-A directory within the `bucket` that the files should be uploaded in to.
+A directory within the `bucket` that the files should be uploaded in to. It should not have a leading or trailing slash.
 
 *Default:* `''`
 
