add documentation on equity index symbols

Author: alexgolec

docs/tda-transition.rst

@@ -85,6 +85,15 @@ Code that manipulates them may need to be updated. ``schwab-py`` provides a
 symbols easier.
 
 
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+Equity index symbols are different
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+
+TDAmeritrade used equity index symbols that ended in ``.X``. For instance, the 
+symbol for the S&P 500 index used to be ``$SPX.X``. Now, these indices are 
+referred to without that suffix, so S&P 500 is just ``$SPX``.
+
+
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 ``schwab-py`` only supports python 3.10 and up
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