Clarify the status of Collection Types in README.

There are bits of code partially handling this, and one is shown in the example, so it's worth clarifying that this doesn't actually work yet to avoid confusion.

Author: apparentlymart

README.md

@@ -95,6 +95,27 @@ it as the `ng-model` of a form element as shown in the HTML example earlier in t
 A more complete example is available in [example/](example/) in the repository, and you can also
 [see the example live](http://saymedia.github.io/angularjs-dynamic-form/example/).
 
+Collection Types
+----------------
+
+The original design for this library called for supporting special "collection types", which
+are fields representing arrays or objects. The intent was that fields could have specifiers
+like `array<string>` which would cause the module to first look for a field type called
+`array` which could then transclude in a field for each item -- in this example the field
+element for `string`. This would allow a single field type to be created for editing arrays
+of any type, delegating to another field type for editing individual elements.
+
+This feature is not yet implemented, although the collection type syntax can be parsed
+and will be ignored. The practical implication of this for the moment is that it is not
+possible to have field types containing `<` and `>` symbols.
+
+The example linked above contains an ``array`` field element, but it is inoperable.
+Support for this may be added in a future version.
+
+In the mean time, arrays of specific types can be supported manually by the caller, by
+creating a type name like `arrayOfString` and then populating that type's field element
+with a UI for adding and removing strings to the array given in `value`.
+
 License
 -------