bpo-11913: Add README.rst to the distutils standard READMEs list (#563)

diff --git a/Lib/distutils/command/sdist.py b/Lib/distutils/command/sdist.py
index 180e286..52eaa15 100644
--- a/Lib/distutils/command/sdist.py
+++ b/Lib/distutils/command/sdist.py
@@ -96,7 +96,7 @@
 
     sub_commands = [('check', checking_metadata)]
 
-    READMES = 'README', 'README.txt'
+    READMES = ('README', 'README.txt', 'README.rst')
 
     def initialize_options(self):
         # 'template' and 'manifest' are, respectively, the names of