relnote: document the use of persistent-nodemap by default for Rust build

Differential Revision: https://phab.mercurial-scm.org/D10498

relnotes/next

@@ -21,6 +21,14 b' Mercurial 5.8.'
     released in May 2019. See `hg help config.format.revlog-compression` for
     details.
 
+  * Mercurial installation built with the Rust parts will now use the
+    "persistent nodemap" feature by default. This feature was introduced in
+    Mercurial 5.4 (May 2020). However Mercurial instalation built without the
+    fast Rust implementation will refuse to interract with them by default.
+    This restriction can be lifted through configuration.
+
+    See `hg help config.format.use-persistent-nodemap` for details
+
 == New Experimental Features ==
 
  * There's a new `diff.merge` config option to show the changes