Add warning about initializing git submodules

[anuejn]

Needed for python dependencies of litex project.

The diff hack is to make the text red. I am open to abandon this, if you don't like it ;)

[xobs]

Enough people have had this problem that it could make sense just to re-enable git fetching in workshop.py and workshop_rgb.py.

These files use lxbuildenv which optionally supports fetching modules via git. The issue is that git isn't a required dependency for the workshop, so we don't enable it.

What do you think? If you edit workshop.py and delete the line that sets skip-git, does it slow down the build noticeably?