docs: note trailing slash handling in README

Closes #175

Co-authored-by: aviadhahami <7353632+aviadhahami@users.noreply.github.com>
Agent-Logs-Url: https://github.com/nodevault/node-vault/sessions/c1ac458b-4e57-4624-90ae-2d76913d4139

Author: Copilot

README.md

@@ -48,6 +48,8 @@ const vault = require('node-vault')({
 });
 ```
 
+> **Note:** Trailing slashes in the `endpoint` URL (e.g. `http://127.0.0.1:8200/`) are automatically stripped to prevent malformed request URIs.
+
 ### Environment Variables
 
 The client reads the following environment variables as defaults: