clarify that one has to clone the mutable-dev-environment repo first
clarify that one has to clone the mutable-dev-environment repo first

I have seen users follow the instructions without realizing they need
to do that first.

Signed-off-by: Vladimir Pantelic <dc9186a06078733915a6fcbab34e59120be2b484@pantelic.net>

pichenettes/mutable-dev-environment