add note about test path to create template

Library/Homebrew/cmd/create.rb

@@ -164,6 +164,9 @@ class FormulaCreator
         # This test will fail and we won't accept that! It's enough to just replace
         # "false" with the main program this formula installs, but it'd be nice if you
         # were more thorough. Run the test with `brew test #{name}`.
+        #
+        # The installed folder is not in the path, so use the entire path to any
+        # executables being tested: `system "#{bin}/program", "--version"`.
         system "false"
       end
     end