Add note about character replacement in the review app URL

README.md

@@ -122,7 +122,8 @@ Thanks to the [Review Apps], the `www-gitlab-com` project supports live reviewin
 of any website changes with every merge request. When a branch is pushed and
 the pipeline is successfully run, you can see a link pointing to the live
 environment in your merge request. The URL will be of the following scheme:
-`<branch-name>.about.gitlab.com`.
+`<branch-name>.about.gitlab.com`.  Note that if you have underscores in your
+branch name, they will be replaced with dashes in the URL.
 Beware that: