Add SQL Server to README.md

SQL Server is now supported via the SqlClient provider.

aspnet/README.md

@@ -36,6 +36,7 @@
 * Npgsql 2.0.12 ([custom build](https://github.com/pdonald/Npgsql2))
 * Entity Framework 6.0.0-alpha3
 * Mongo C# Driver 1.8.1
+* SqlClient Data Provider for SQL Server
 
 **Developer Tools**