belt-clamp-readme.scad missing from repo

Howdy!

https://github.com/reprappro/Mendel/blob/master/Openscad/x-carriage-readme.scad requires "belt-clamp-readme.scad," but this file isn't in the repo.

So either x-carriage-readme.scad is deprecated/broken and should be removed/updated, or belt-clamp-readme.scad is required and should be added.

Thanks!
- Patrick
