Add note on functional tests' requirements (#8134)

Author: loic-sharma

tests/README.md

@@ -2,6 +2,10 @@
 
 Tests for the NuGet Gallery
 
+## Prerequisites
+
+To run the functional tests, you will need to install Visual Studio 2015 Enterprise. 
+
 ## Projects
 
 Unit Tests
@@ -19,4 +23,4 @@ Web UI Tests
 *  NuGetGallery.WebUITests.P0
 *  NuGetGallery.WebUITests.P1
 *  NuGetGallery.WebUITests.P2
-*  NuGetGallery.WebUITests.ReadOnlyMode
+*  NuGetGallery.WebUITests.ReadOnlyMode