Perhaps the following? Assume this might belong on http://docs.travis-ci.com/user/addons/

## Overriding the default script

An add-on can define its own stages, to be run before the built-in stages defined in `travis-build`, and configured by the project's `.travis.yml` file. For example, the hosts add-on has a `setup` stage, which edits the `/etc/hosts` file per the add-on configuration, before running the built in `setup` stage.

This also applies to the main script stage. In some cases, it may be desirable to override the script stage, rather than to prepend to it. For example, an alternate analysis tool can be triggered in place of the normal CI flow. One example of this is the `coverity_scan` add-on. By default, a commit to the `coverity_scan` branch triggers the [Coverity Scan](http://scan.coverity.com) analysis to be run instead of running tests. 

The method `Travis::Build::Script#delete_script` in `lib/travis/build/script.rb` is called by the `coverity_scan` add-on in to consume the built-in script stage.