Add warning for legacy version

Author: Matthiee

README.md

@@ -1,3 +1,6 @@
+> [!WARNING]  
+> This contains the API bindings for the legacy version of FMP. This does not work for newer accounts, see https://github.com/MatthiWare/FinancialModelingPrep.NET/issues/145.
+
 # FinancialModelingPrep.NET
 
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