Added PCL section in readme

readme.md

@@ -142,6 +142,13 @@ Xamarin
 
 [Xamarin Readme](/readme-Xamarin.md)
 
+PCL
+---
+
+The initial dependencies to Microsoft.Net.Http, Microsoft.Bcl and Microsoft.Bcl.Build have been removed from the original package to avoid conflicts in environments where they cannot be installed (like Xamarin that supports those functionalities). In case your environment does not support Microsoft.Net.Http.HttpClient or async/await please add mentioned previously packages manually.
+
+In case you do not need to install any additional libraries make sure that you have added reference to Microsot.System.Net assembly.
+
 Documentation
 ===