Update README.md (#334)

README.md

@@ -47,7 +47,7 @@ apt install texlive-latex-base
 You will need several input files:
 
 -   Exported transaction history from Schwab in CSV format since the beginning.
-    Or at least since you first acquired the shares, which you were holding during the tax year.
+    Or at least since you first acquired the shares, which you were holding during the tax year. Schwab only allows to download transaction for the last 4 years so keep it safe. After that you may need to restore transactions from PDF statements.
     [See example](https://github.com/KapJI/capital-gains-calculator/blob/main/tests/test_data/schwab_transactions.csv).
 -   Exported transaction history from Schwab Equity Awards (e.g. for Alphabet/Google employees) since the beginning. These are to be downloaded in JSON format. Instructions are available at the top of the [parser file](../main/cgt_calc/parsers/schwab_equity_award_json.py).
 -   Exported transaction history from Trading 212.