added signature warning

Per #554.

Author: shannona

09_3_Testing_a_Bitcoin_Script.md

@@ -182,6 +182,8 @@ btcdeb> stack
 ```
 Using these commands can make it easier to see what's going on and where you are.
 
+> :warning: `btcdeb` is much more complex to use if you are trying to verify signatures. See [Signature Checking with btcdeb](https://github.com/bitcoin-core/btcdeb/blob/master/doc/btcdeb.md#signature-checking). This is true for any script testing, so we don't suggest it if you're trying to verify an `OP_CHECKSIG` or an `OP_CHECKMULTISIG`.
+
 ## Test a Script Online
 
 There are also a few web simulators that you can use to test scripts online. They can be superior to a command-line tool by offering a more graphical output, but we also find that they tend to have shortcomings.