Add note to Readme about Angular 6 usage

lib/msal-angular/README.md

@@ -4,7 +4,6 @@ Microsoft Authentication Library for Angular Preview
 
 The MSAL library preview for Angular is a wrapper of the core MSAL.js library which enables Angular(4.3 to 5) applications to authenticate enterprise users using Microsoft Azure Active Directory (AAD), Microsoft account users (MSA), users using social identity providers like Facebook, Google, LinkedIn etc. and get access to [Microsoft Cloud](https://cloud.microsoft.com) OR  [Microsoft Graph](https://graph.microsoft.io).
 
-
 [![Build Status](https://travis-ci.org/AzureAD/microsoft-authentication-library-for-js.png?branch=dev)](https://travis-ci.org/AzureAD/microsoft-authentication-library-for-js)
 
 ## Important Note about the MSAL Angular Preview
@@ -21,6 +20,8 @@ The msal-angular package is available on NPM:
 
 Before using MSAL, register your application in [Azure AD v2.0 portal](https://apps.dev.microsoft.com/) to get your clientID. As part of the registration, you will also need to add the Web platform, check the "Implicit Flow" checkbox, and add the redirectURI to your application.
 
+> NOTE: To use MSAL Angular with Angular 6, please install the `rxjs-compat` NPM module for now. In future, we plan to add support for Angular 6 in the library and you can track this [issue](https://github.com/AzureAD/microsoft-authentication-library-for-js/issues/421) for more information.
+
 #### 1. Include and initialize the MSAL module in your app module.
 Import MsalModule into app.module.ts. To initialize MSAL module you are required to pass the clientID of your application which you can get from the application registration.