get rid of the tmpl extension from template files

.. also add a readme explaining that files in templ directory should not
be opened with visual studio.

template files are already in a tmpl directory which is enough to
identify them as template files without having the templ extension.
however it's possible the user won't understand that? so i added a
readme explaining that they are template files.

projects/Windows/tmpl/README.txt

@@ -0,0 +1,6 @@
+The files in this directory are not meant to be opened with Visual Studio.
+
+This directory contains template files that are used to generate project files
+for legacy versions of Visual Studio 2010, 2012 and 2013. To generate the
+files run generate.bat in the projects directory and refer to that directory's
+README.