Add comment regarding updating the peg generator when adding tokens

Author: alexprengere

Grammar/Tokens

@@ -1,3 +1,7 @@
+# When adding new tokens, remember to update as well the peg generator in
+# Tools/peg_generator/pegen/parser_generator.py
+# This will ensure that older versions of Python can generate a Python parser
+# using "python -m pegen python <GRAMMAR FILE>".
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