Add change note.

Here's the improvement for the greenlet pools:

| Benchmark          | 36_pool_master | 36_pool_opts                |
+--------------------+----------------+-----------------------------+
| imap_unordered_seq | 803 us         | 686 us: 1.17x faster (-15%) |
| imap_unordered_par | 445 us         | 389 us: 1.14x faster (-13%) |
| imap_seq           | 793 us         | 729 us: 1.09x faster (-8%)  |
| imap_par           | 407 us         | 398 us: 1.02x faster (-2%)  |
| map_seq            | 715 us         | 293 us: 2.44x faster (-59%) |
| map_par            | 388 us         | 199 us: 1.96x faster (-49%) |

Not significant (2): apply; spawn

CHANGES.rst

@@ -42,6 +42,9 @@ Enhancements
 - Hub objects now include the value of their ``name`` attribute in
   their repr.
 
+- Pools for greenlets and threads have lower overhead, especially for
+  ``map``. See :pr:`1153`.
+
 Monitoring and Debugging
 ------------------------