Document old variant computation once develop/#297 is accepted

[rsasse]

Need to document the
TAMARIN_NO_MAUDE_VARIANTS
environment variable which can be set to use the old, Tamarin-internal variant computation. This might be needed for reproduction of older examples.

[rsasse]

Essentially, one sets the variable by executing
export TAMARIN_NO_MAUDE_VARIANTS=1
the success of which can be checked by calling
echo $TAMARIN_NO_MAUDE_VARIANTS
which should return 1 and leads to any subsequent Tamarin calls (in that current shell!) to use the old variant computation. Use unset to remove the variable and go back to Maude-based variants.

This documentation should be added before the next release.