Dev/annagrin/explicit notnull transition description (microsoft#722)

* Added c++17 test configurations for clang5.0 and clang6.0

* Improved description how to make transition to explicit not_null constructor

samples/gsl_transition

@@ -40,6 +40,16 @@ namespace gsl_helpers
 // To make the transition:
 //
 // - replace all occurences of gsl::not_null in your code by sloppy_not_null
+//   variant: rename gsl::not_null by NotNull by including the following in your code,
+//   foe example, in a common include file:
+//
+//   template<typename T>
+//   using NotNull = gsl::not_null<T>;
+//
+//   compile using old version of GSL
+//   change GSL version and replace gsl::not_null by gsl_helpers::sloppy_not_null
+//   in the added code lines above
+// 
 // - compile - compilation should be successful
 // - replace some sloppy_not_nulls by gsl::not_null, fix compilation erros, 
 //   redesign as needed, compile and test