update main root license to be txt (#888)

fixes: https://github.com/Azure/azure-sdk-for-cpp/issues/887

README.md

@@ -71,7 +71,7 @@ Many people all over the world have helped make this project better.  You'll wan
 Security issues and bugs should be reported privately, via email, to the Microsoft Security Response Center (MSRC) <secure@microsoft.com>. You should receive a response within 24 hours. If for some reason you do not, please follow up via email to ensure we received your original message. Further information, including the MSRC PGP key, can be found in the [Security TechCenter](https://www.microsoft.com/msrc/faqs-report-an-issue).
 
 ### License
-
+<!-- Allow LICENSE to be changed to LICENSE.txt, then this comment will be removed 
 Azure SDK for C++ is licensed under the [MIT](https://github.com/Azure/azure-sdk-for-cpp/blob/master/LICENSE) license.
-
+-->
 ![Impressions](https://azure-sdk-impressions.azurewebsites.net/api/impressions/azure-sdk-for-cpp%2FREADME.png)