README: Add 'setuptools_scm' to 'pyproject.toml'

Ensure 'setuptools_scm' is defined in both 'pyproject.toml' and
'setup.cfg'. Newer pips will use the former (and skip the latter as
already resolved by the time setup_requires are evaluated), and older
pips that don't know about 'pyproject.toml' can keep using
'setup_requires'.

Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin

README.rst

@@ -110,7 +110,7 @@ version of ``setuptools``:
 
     # pyproject.toml
     [build-system]
-    requires = ["setuptools>=30.3.0", "wheel"]
+    requires = ["setuptools>=30.3.0", "wheel", "setuptools_scm"]
 
 For more information, refer to the `setuptools issue #1002
 <https://github.com/pypa/setuptools/issues/1002>`_.