Add documentation about integration points

[jcoyne]

So that some developer doesn't try to remove them 😊

[jrochkind]

I had thought maybe these woudl be going away with the big switch to view components instead of helper methods?

But I like not doing so, at least not right away.

I wonder if returning some of the helper methods that are also commonly used as integration points by plugins, but have been removed as part of view component integration, is a possible solution to missing integration points? Maybe not though. Just trying to keep being alert to possible ways to restore the integration points the plugins need that have been removed.