Note in benchmark README

Author: spookylukey

tools/benchmarks/README.md

@@ -11,8 +11,15 @@ You can also run them using py.test with extra args:
 
     $ py.test --benchmark-warmup=on runtime.py -k interpolation
 
-To profile the benchmark suite, we recommend py-spy as a
-good tool. Install py-spy: https://github.com/benfred/py-spy
+The “plural form” tests are the cases where GNU gettext performs most
+favourably, partly because it uses a much simpler (and incorrect) function for
+deciding plural forms, while we use the more complex ones from CLDR. You can
+exclude those by doing:
+
+    $ py.test --benchmark-warmup=on runtime.py -k 'not plural'
+
+To profile the benchmark suite, we recommend py-spy as a good tool. Install
+py-spy: https://github.com/benfred/py-spy
 
 Then do something like this to profile the benchmark. Depending on your
 platform, you might need to use `sudo`.