docs: add troubleshooting guide for non-standard token approvals

Author: iborazzi

README.md

@@ -7,3 +7,9 @@ Welcome to the official documentation for Ink, Kraken's dedicated DeFi chain.
 * Bridge assets and acquire testnet funds
 * Explore tutorials and start building onchain!
 
+## Troubleshooting: Non-Standard ERC-20 Tokens & Approvals
+If you encounter an "internal error" or transaction failure when attempting to revoke token approvals (such as calling approve(spender, 0) on certain mainnet tokens):
+* **Token Contract Implementation:** Some non-standard ERC-20 tokens may throw errors if approve is called directly to 0 while an active non-zero allowance exists.
+* **Workarounds:** 
+  * Try setting the allowance to a lower intermediate value before resetting to 0.
+  * Verify token contract details via Blockscout to ensure compliance with standard interfaces.