Added breaking changes documentation

Author: robdmoore

BREAKING_CHANGES.md

@@ -1,6 +1,26 @@
 AzureWebFarm.OctopusDeploy Breaking Changes
 ===========================================
 
+Version 2.0.0
+-------------
+
+### OctopusDeploy 3 Support
+
+We now use the API client library from Octopus Deploy 3 - it's likely that this would break Octopus Deploy 2 so use with caution ifi you are still on 2 or use the 1.x series of AzureWebFarm.OctopusDeploy.
+
+### `WebFarmRole.Run()` signature changed
+
+WebFarm role now takes a `CancellationToken` and returns a `Task` so it can be awaited. This makes it easier to support other code running side-by-side in your role and exert more control over the `Run` method.
+
+If you want the behaviour to stay the same as before then put the following in your `WebRole` (RoleEntryPoint) class `Run` method:
+
+```c#
+        public override void Run()
+        {
+            Task.Run(() => _webFarmRole.Run(new CancellationTokenSource().Token)).Wait();
+        }
+```
+
 Version 1.2.0
 -------------