Update README with G115 rule description

Signed-off-by: Cosmin Cojocar <cosmin@cojocar.ch>

README.md

@@ -139,6 +139,7 @@ directory you can supply `./...` as the input argument.
 - G112: Potential slowloris attack
 - G113: Usage of Rat.SetString in math/big with an overflow (CVE-2022-23772)
 - G114: Use of net/http serve function that has no support for setting timeouts
+- G115: Potential integer overflow when converting between integer types
 - G201: SQL query construction using format string
 - G202: SQL query construction using string concatenation
 - G203: Use of unescaped data in HTML templates