examples: include a sample of how to use black with fix

It's commented out for now since my patch hasn't landed, but we can
uncomment it when that lands.

Differential Revision: https://phab.mercurial-scm.org/D6974

contrib/examples/fix.hgrc

@@ -4,3 +4,7 b' clang-format:pattern = (**.c or **.cc or'
 
 rustfmt:command = rustfmt {rootpath}
 rustfmt:pattern = set:**.rs
+
+# We use black, but currently with https://github.com/psf/black/pull/826 applied.
+# black:command = black --skip-string-normalization
+# black:pattern = set:**.py - hgext/fsmonitor/pywatchman/** - mercurial/thirdparty/** - "contrib/python-zstandard/**"