Add a troubleshooting section with Postgres tips

README.md

@@ -96,3 +96,11 @@ bundle exec foreman start
 ```
 Now you can go to http://localhost:3000 in your browser.
+
+## Troubleshooting
+
+### Rails cannot connect to Postgres
+
+- Check if foreman is running in the gitlab-development-kit directory.
+- Check for custom Postgres connection settings defined via the environment; we
+  assume none such variables are set. Look for them with `set | grep '^PG_'`.