README: fix install guide.

Author: ryanelian

README.MD

@@ -5,11 +5,17 @@
 
 ## Install
 
-```powershell
-npm install aspnet-validation es6-promise
+```
+npm install aspnet-validation
+```
+
+or
+
+```
+yarn add aspnet-validation
 ```
 
-> aspnet-validation uses [Promise API](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Promise), which is not supported in Internet Explorer. It is recommended to use [promise-polyfill](https://github.com/taylorhakes/promise-polyfill) or [ts-polyfill](https://github.com/ryanelian/ts-polyfill) or [core-js](https://github.com/zloirock/core-js) to resolve this issue...
+> aspnet-validation uses [Promise API](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Promise), which is not supported in Internet Explorer. **It is recommended to use [promise-polyfill](https://github.com/taylorhakes/promise-polyfill) or [ts-polyfill](https://github.com/ryanelian/ts-polyfill) or [core-js](https://github.com/zloirock/core-js) to resolve this issue...**
 
 Alternatively, download these: