doc: update release notes for v0.4.0 release (#284)

Author: coryan

CHANGELOG.md

@@ -1,6 +1,23 @@
 # Changelog
 
-## v0.4.0 - TBD
+## v0.5.0 - TBD
+
+## v0.4.0 - 2021-02
+
+* feat: use structured logging for exceptions (#278)
+* feat: convert storage Pub/Sub events (#273)
+* feat: how-to guide using Cloud Build for builder (#267)
+* chore: update to pack v0.17.0 (#263)
+* feat: implement code for Cloud Spanner tutorial (#239)
+* feat: implement code for Cloud Bigtable tutorial (#238)
+* doc: this library is now published in vcpkg (#237)
+* feat: fix compilation problems on MSVC (#233)
+* doc: new guide to deploy Pub/Sub function to Cloud Run (#234)
+* doc: new guide to deploy on Cloud Run (#232)
+* doc: describe how to create a Docker image (#212)
+* doc: describe how to run a local server (#209)
+* We implemented many other examples, but this changelog is
+  too long already.
 
 ## v0.3.0 - 2021-02