Add dep caching to the CHANGELOG as experimental

CHANGELOG.asciidoc

@@ -43,3 +43,7 @@
 
 2023/05/15: Protobuff compilation with `gpb` now uses the
             `{use_packages, true}` option.
+
+2023/05/15: Experimental support for caching git and
+            hex dependencies has been added. Set
+            `CACHE_DEPS=1` to enable.