Add technique for preventing dumbness.

doc/merger-intro.md

@@ -103,6 +103,14 @@ thumb:
 
 The stages are discussed in more detail in the primary [README.md](../README.md).
 
+## Preventing accidental pushes to the main repository
+
+In the past you might have accidentally tried pushing to the main repository and it would have warned you that you don't have access. You may want to set up your git remote for the main repository as suggested at https://stackoverflow.com/a/7556269/8332458:
+
+`git remote set-url --push origin notallowed`
+
+where origin is the label you've used for the main civicrm/civicrm-core repository.
+
 ## Communication media
 
 Two channels are generally important for anyone who merges: