add announcement for mambabuild

Author: wolfv

src/user/announcements.rst

@@ -8,6 +8,12 @@ Our announcements are published to an RSS feed `here <https://conda-forge.org/do
 2021
 ----
 
+:2021-10-20: conda-forge now uses mambabuild as default
+
+    conda-forge now uses `mamba <https://github.com/mamba-org/mamba>`_ during the build process (via ``conda mambabuild`` of the `boa <https://github.com/mamba-org/boa>`_ project). This was 
+    changed in `conda-smithy 3.13.0 <https://github.com/conda-forge/conda-smithy/blob/main/CHANGELOG.rst#v3130>`_
+    and should automatically apply when re-rendering.
+
 :2021-10-13: GCC 10 and clang 12 as default compilers for Linux and macOS
 
     These compilers will become the default for building packages in conda-forge.