Changed constant to view and more expressive comment

contracts/crowdsale/Crowdsale.sol

@@ -64,8 +64,8 @@ contract Crowdsale {
     buyTokens(msg.sender);
   }
 
-  // Override this function to create logic for periodization
-  function getTokenAmount(uint256 weiAmount) internal constant returns(uint256) {
+  // Override this method to have a way to add business logic to your crowdsale when buying
+  function getTokenAmount(uint256 weiAmount) internal view returns(uint256) {
     return weiAmount.mul(rate);
   }