[add] HTTP/2 notice

Author: i4004

README.md

@@ -10,6 +10,8 @@
 
 Etcd based configuration provider for Microsoft.Extensions.Configuration.
 
+    (!) Keep in mind that starting from v2 it will work only under Windows 10 or Windows Server 2016 (or later) because of internal Grpc client uses HTTP/2 protocol for communication (https://docs.microsoft.com/en-us/answers/questions/310032/2012-r2-net-core-http2.html).
+
 ## Quick start
 
 ### HTTP