Document that rpmbuild is required for bdist_rpm (#1547)

Document that rpmbuild is required for bdist_rpm

Refs #1456

changelog.d/1456.doc.rst

+Documented that the ``rpmbuild`` packages is required for the ``bdist_rpm`` command.

docs/setuptools.txt

@@ -1333,6 +1333,10 @@ Creating System Packages
     ``bdist_deb`` command is an example of a command that currently requires
     this kind of patching to work with setuptools.
 
+    Please note that building system packages may require you to install
+    some system software, for example ``bdist_rpm`` requires the ``rpmbuild``
+    command installed.
+
     If you or your users have a problem building a usable system package for
     your project, please report the problem via the mailing list so that
     either the "bdist" tool in question or setuptools can be modified to