updating details on why this restriction was added

Author: weaverryan

CHANGELOG.md

@@ -3,7 +3,9 @@
 ## 0.13.0
 
  * The `setManifestKeyPrefix()` method now forbids the key to
-   start with a `/` - #105.
+   start with a `/` - #105. This is purposely restricting to
+   avoid users doing this by accident (normally the manifest key
+   prefix does not start with `/` when calculated automatically).
 
 ## 0.12.0