Update letsencrypt note to include drop all rules

docs/tre-admins/setup-instructions/manual-deployment.md

@@ -31,6 +31,12 @@ make letsencrypt
 !!! info
     If you're using Codespaces, you'll encounter a bug when trying to run `make letsencrypt` where the incorrect IP will be whitelisted on the storage account and Codespaces won't be able to upload the test file due to a 403 error. The workaround until this is fixed is to temporarily disable the firewall on your `stweb{TRE_ID}` storage account before running the script, then re-enable afterwards.
 
+    The script can do this for you if you run it with setting a LETSENCRYPT_DROP_ALL_RULES environment variable to `1`:
+
+    ```bash
+    LETSENCRYPT_DROP_ALL_RULES=1 make letsencrypt
+    ```
+
 ## Validate the deployment
 
 ### Using curl