docs: add note about libbson automatic bundling.

Author: Christian Hergert

README.rst

@@ -11,6 +11,7 @@ There are absolutely no guarantees of API/ABI stability at this point.
 But generally, we won't break API/ABI unless we have good reason.
 
 mongo-c-driver depends on `Libbson <https://github.com/mongodb/libbson>`_.
+Libbson will automatically be built if you do not have it installed on your system.
 
 If you are looking for the legacy C driver, it can be found in the
 `legacy branch <https://github.com/mongodb/mongo-c-driver/tree/legacy>`_.