Add a very short summary

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/docs/3.0-migration-guide.md b/docs/3.0-migration-guide.md
index 7874909..f64b209 100644
--- a/docs/3.0-migration-guide.md
+++ b/docs/3.0-migration-guide.md
@@ -9,6 +9,12 @@
 Here's the list of breaking changes; each entry should help you answer these
 two questions: (1) am I affected? (2) if yes, what's my migration path?
 
+The changes are detailed below, and include:
+- Removal of many insecure / obsolete features
+- Tidying up of configuration options (including removing some less useful options)
+- Changing function signatures (e.g., adding return codes or extra parameters); introducing const to arguments.
+- Removal of functions marked as deprecated in 2.x
+
 Introduce a level of indirection and versioning in the config files
 -------------------------------------------------------------------