Note undesired behavior from setting enabled: true in root project

From dbt Slack:

> A user had the following config in dbt_project.yml:
> ```
> models:
> ...
> snowplow:
> enabled: true
> ```
> Setting `enabled: true` in the root project overrides the dynamic, target-dependent enabled values set from within the snowplow package. Removing that line made this magically work again.
>
> I don't think there's anything we could or should do to "lock" those enabled values within a package, since it's expected (and good) behavior that the user can override locally. What I will do is document that setting a top-level `enabled` config can have surprising results.

Reminder to self to add a note to the README :)