Update documentation

Author: mickael-menu

CHANGELOG.md

@@ -14,7 +14,8 @@ All notable changes to this project will be documented in this file. Take a look
 
 ### Changed
 
-The Readium Swift toolkit now requires a minimum of iOS 13.
+* The Readium Swift toolkit now requires a minimum of iOS 13.
+* Plenty of completion-based APIs were changed to use `async` functions instead.
 
 #### Shared
 

Documentation/Migration Guide.md

@@ -2,6 +2,13 @@
 
 All migration steps necessary in reading apps to upgrade to major versions of the Swift Readium toolkit will be documented in this file.
 
+## Unreleased
+
+### Async APIs
+
+Plenty of completion-based APIs were changed to use `async` functions instead. Follow the deprecation warnings to update your codebase.
+
+
 ## 3.0.0-alpha.1
 
 ### R2 prefix dropped