Showing the mesh contents in the readme

I would be keen to somehow highlight the tags and how they appear in the mesh in the readme.

Perhaps after this line in the readme

```The resulting .msh file should have three physical groups named for tags in it. The in_contact group should include the faces from both the shell and the insert.```

We could insert a snippet of a mesh that has the tags highlighted.

perhaps and expandable snippet like this, which I've had working in readme.md files on github before, looks like it doesn't work in issues

Expand - See the resulting mesh?

       contents of the mesh here with highlighted tags

        

I think this would clearly show

This is something I can take on when I get a few mins space, just making an issue so i don't forget