docs: update release notes

Author: ellemouton

docs/release-notes/release-notes-0.20.0.md

@@ -127,8 +127,15 @@ reader of a payment request.
   to improve readability and maintainability of the code.
 
 ## Breaking Changes
+
 ## Performance Improvements
 
+* Let the channel graph cache be populated asynchronously on startup. While the 
+  cache is being populated, the graph is still available for queries, but
+  all read queries will be served from the database until the cache is fully 
+  populated. This new behaviour can be opted out of via the new 
+  `--db.sync-graph-cache-load` option.
+
 ## Deprecations
 
 ### ⚠️ **Warning:** The following RPCs will be removed in release version **0.21**: