Add message to caution that SafeMath.sol is for Solidity 0.8

contracts/math/SafeMath.sol

@@ -2,6 +2,10 @@
 
 pragma solidity ^0.8.0;
 
+// CAUTION
+// This version of SafeMath should only be used with Solidity 0.8 or later,
+// because it relies on the compiler's built in overflow checks.
+
 /**
  * @dev Wrappers over Solidity's arithmetic operations.
  */