Add explaining comment for the web3Provider type

src/types.ts

@@ -281,5 +281,9 @@ export interface ContractEventEmitter {
     watch: (eventCallback: EventCallback) => void;
     stopWatchingAsync: () => Promise<void>;
 }
-
+/**
+ * We re-export the `Web3.Provider` type specified in the Web3 Typescript typings
+ * since it is the type of the `provider` argument to the `ZeroEx` constructor.
+ * It is however a `Web3` library type, not a native `0x.js` type.
+ */
 export type Web3Provider = Web3.Provider;