wrote some release notes

ReleaseNotes.md

@@ -1,6 +1,43 @@
 ### New in 0.21.0 (released 2016/07/29)
 
-**TODO**
+**Features**
+
+This release adds support across Octokit.net for providing the repository Id
+rather than a name/owner pair. The repository Id does not change when transferring
+ownership of a repository, and is more robust for API callers. This work
+was lead by @dampir as part of Google Summer of Code 2016.
+
+ - Added new fields for Deployment and DeploymentStatus preview API - #1365 via @ErikSchierboom
+ - Added new fields for signature verification to Git Data Tag API - #1420 via @Sarmad93
+ - Added new fields for GitHub Pages preview API - #1421 via @dampir
+
+**Fixes**
+
+ - Fix serialization of enum value attributes - #1402 via @maddin2016
+ - Fix searching for repositories with underscore in name - #1418 via @dsplaisted, @shiftkey
+
+**Other**
+
+ - Clarified obsolete warnings for Protected Branch preview API - #1428 via @ryangribble
+ - Remove Obsolete items - #1422 via @ryangribble
+
+**Breaking Changes**
+
+After a long grace period, #1422 has removed these obsoleted members. These features
+exist in other parts of the API surface:
+
+ - `I(Observable)GitHubClient.Release`
+ - `I(Observable)GitHubClient.Notification`
+ - `I(Observable)GitHubClient.GitDatabase`
+ - `I(Observable)GitHubClient.SshKey`
+ - `I(Observable)GitHubClient.Repository.RepositoryComments`
+ - `I(Observable)GitHubClient.Repository.CommitStatus`
+ - `I(Observable)GitHubClient.Repository.RepoCollaborators`
+ - `I(Observable)GitHubClient.Repository.Commits`
+
+This method has also been removed, but is no longer supported through the API: 
+
+ - `I(Observable)GitHubClient.Authorization.RevokeAllApplicationAuthentications()`
 
 ### New in 0.20.0 (released 2016/06/15)