relnotes: advertize the possibility to use rust

I think the rust work may have been mentioned in the release notes,
but if so only in passing, and not as an invitation to try it out.

I think the next version is a decent time to do this, because the rust
doesn't come with performance regressions AFAIK, speeds up status
noticeably when it applies, which is the case for most invocations of
status, and doesn't have the undesirable restriction of regex around
empty patterns anymore.

I am cheating a bit, because I'm giving numbers for `hg status` in
mozilla-central, but they have one hgignore pattern that uses
lookaround, ".vscode/(?!extensions\.json|tasks\.json", which I took
out as it would cause a fallback to python when unknown files are
requested. But it seems that they could express their hgignore
differently if they were so inclined.

Not sure if there are limitation other than linux-only that I am
not thinking of but would be worth mentioning upfront, to avoid
disappointing users?

Differential Revision: https://phab.mercurial-scm.org/D8604

relnotes/next

@@ -3,6 +3,12 b''
 
 == New Experimental Features ==
 
+ * The core of some hg operations have been (and are being)
+   implemented in rust, for speed. `hg status` on a repository with
+   300k tracked files goes from 1.8s to 0.6s for instance.
+   This has currently been tested only on linux, and does not build on
+   windows. See rust/README.rst in the mercurial repository for
+   instructions to opt into this.
 
 == Backwards Compatibility Changes ==