Add HACK comments

packages/contract-wrappers/src/contract_wrappers/erc20_proxy_wrapper.ts

@@ -49,6 +49,8 @@ export class ERC20ProxyWrapper extends ContractWrapper {
         const contractAddress = this._getContractAddress(artifacts.ERC20Proxy, this._contractAddressIfExists);
         return contractAddress;
     }
+    // HACK: We don't want this method to be visible to the other units within that package but not to the end user.
+    // TS doesn't give that possibility and therefore we make it private and access it over an any cast. Because of that tslint sees it as unused.
     // tslint:disable-next-line:no-unused-variable
     private _invalidateContractInstance(): void {
         delete this._erc20ProxyContractIfExists;