Add a note about push to secondary using HTTP

doc/administration/geo/replication/usage.md

@@ -25,3 +25,11 @@ remote:   ssh://git@primary.geo/user/repo.git
 remote:
 Everything up-to-date
 ```
+
+NOTE:
+If you're using HTTPS instead of [SSH](../../../ssh/README.md) to push to the secondary,
+you can't store credentials in the URL like `user:password@URL`. Instead, you can use a
+[`.netrc` file](https://www.gnu.org/software/inetutils/manual/html_node/The-_002enetrc-file.html)
+for Unix-like operating systems or `_netrc` for Windows. In that case, the credentials
+will be stored as a plain text. If you're looking for a more secure way to store credentials,
+you can use [Git Credential Storage](https://git-scm.com/book/en/v2/Git-Tools-Credential-Storage).