add section on forking for contributing.md (#678)

* add section on forking for contributing.md

* fix cap

CONTRIBUTING.md

@@ -7,6 +7,19 @@ This document will guide you through the process.
 Please visit [https://cla.microsoft.com/](https://cla.microsoft.com/) and sign the Contributor License
 Agreement.  You only need to do that once. We can not look at your code until you've submitted this request.
 
+## Fork
+
+Fork the project [on GitHub](https://github.com/AzureAD/microsoft-identity-web) and check out
+your copy.
+
+Example for Microsoft Identity Web:
+
+```
+$ git clone [email protected]:username/microsoft-identity-web.git
+$ cd microsoft-identity-web
+$ git remote add upstream [email protected]:AzureAD/microsoft-identity-web.git
+```
+
 ## Setup, Building and Testing
 
 Please see the [Build & Run](https://github.com/AzureAD/microsoft-identity-web/wiki/build-and-test) wiki page.