fix(topupcredit.sol, subsidy.circom): add comments for upper bound of…

… constants

(1) M in subsidy calculation (2) hard cap of voice credit

circuits/circom/subsidy.circom

@@ -17,6 +17,7 @@ template SubsidyPerBatch (
     intStateTreeDepth,
     voteOptionTreeDepth
 ) {
+    // keep MM < 2 ** 196 to avoid overflow: https://hackmd.io/@chaosma/H1_9xmT2K
     var MM = 50; // protocol params should be consistent with MaciState.ts
     var WW = 4; // number of decimal in float representation, should consist with MaciState.ts
     var NN = 64; // maximum width of bits (10**NN < 2**253) in float division

contracts/contracts/TopupCredit.sol

@@ -6,6 +6,8 @@ import "@openzeppelin/contracts/access/Ownable.sol";
 
 contract TopupCredit is ERC20, Ownable {
     uint8 private _decimals = 1;
+    // keep the maximum amount less than 2**126 < sqrt(field_size)
+    // to avoid overflow
     uint256 MAXIMUM_AIRDROP_AMOUNT = 100000 * 10**_decimals;
 
     constructor() ERC20("TopupCredit", "TopupCredit") {