Merge pull request #86 from ably/10-document-testing-approach

[ECO-5385] Document the approach for using ably-cocoa internals in tests

Author: lawrence-forooghian

CONTRIBUTING.md

@@ -123,6 +123,10 @@ Example:
 // @specNotApplicable CHA-EX3a - Our API does not have a concept of "partial options" unlike the JS API which this spec item considers.
 ```
 
+#### Using ably-cocoa internals in tests
+
+Some of our integration tests require access to ably-cocoa internals that are not exposed via `_AblyPluginSupportPrivate`, for example to inject protocol messages. Since, unlike the plugin implementation, the test suite does not require access to a stable private API (as it will never be compiled by end users and we're in control of which version of ably-cocoa gets used for testing the plugin), we just directly import ably-cocoa's internal APIs in the test suite using `import Ably.Private`.
+
 ## Developing ably-cocoa alongside this plugin
 
 The quickest way to edit ably-cocoa is to use `swift package edit ably-cocoa --path ably-cocoa`, which will give you a Git checkout of ably-cocoa at `ably-cocoa`. To edit ably-cocoa using Xcode, you will then need to open `ably-cocoa/Package.swift` _separately_ (making sure to close any other LiveObjects Xcode windows, else Xcode will not let you edit it).