Add a news item.

* NEWS: Document the previous commit.

NEWS

+Recent changes:
+  - Add an upper bound on the permitted class number in ECPP, to avoid
+    choosing discriminants for which class polynomials cannot be computed
+    in reasonable time and with reasonable memory.
+
 Changes in version 0.4.2 ("Fitzebohnen"), released in May 2023:
   - If the environment variable CM_ECPP_TMPDIR is set, write checkpoint
     files during the second phase of ECPP while factoring class