Add a document describing the now correct handling of extension

methods in alias-referenced assemblies.
Fixes #1079

docs/compilers/Extension Methods in Aliased Assemblies.md

+Extension Methods in the Global Namespace
+=========================================
+
+If you create a compilation in which an imported library is assigned an alias (instead of the default "global" alias), the Roslyn compilers do not automatically see extension methods in the (unnamed top-level) global namespace in that library. You would have to import the library in your code using the alias for these methods to be treated as extension methods.
+
+Previous versions of the compiler would treat such methods as extension methods despite not being imported.
+
+See also https://github.com/dotnet/roslyn/issues/1079.