Add CLA completion link in the complex PR chapter (dotnet#2237)

I had to complete the CLA and it was difficult to know what the procedure is. On google, you'll find multiple links and posts, all wrong or obsolete : https://www.google.fr/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=CLA+microsoft+github&start=0

CONTRIBUTING.md

@@ -13,6 +13,7 @@ You'll need a basic understanding of [Git and GitHub.com](https://guides.github.
 * Open an [issue](https://github.com/aspnet/Docs/issues/new) describing what you want to do, such as change an existing article or create a new one. Wait for approval from the ASP.NET documentation team before you invest much time. 
 * Fork the [aspnet/Docs](https://github.com/aspnet/Docs/) repo and create a branch for your changes.
 * Submit a pull request (PR) to master with your changes.
+* If your PR has the label 'cla-required' assigned, [complete the Contribution License Agreement (CLA)](https://cla2.dotnetfoundation.org/)
 * Respond to PR feedback.
 
 For an example where this process led to publication of a new article, see [issue 67](https://github.com/dotnet/docs/issues/67) and [pull request 798](https://github.com/dotnet/docs/pull/798) in the .NET Core repository. The new article is [Documenting your code](https://docs.microsoft.com/dotnet/articles/csharp/codedoc).