`--frozen` to expect no changes in README

[Profpatsch]

This is another one for CI:

mdsh --frozen should check whether the new README is exactly the same as the old one, and fail if they are not the same.

We can achieve something similar with a trivial:

git diff --exit-code

after running mdsh, but that picks up all files that have been modified by the commands in the md file, which might or might not be what the user wants.

Alternatively, a wrapper script like

INPUT="$1"
tmp=$(mktemp)
cp "$INPUT" "$tmp"
mdsh "$INPUT"
diff "$tmp" "$INPUT"

might also be enough for most use cases.

I’m not sure whether --frozen is necessary in the end, but if it isn’t the best practices should be documented.