Add warning that #process destroys its input

The `#process` method works by progressively consuming the input sexp, so
if you need to preserve the input value for other purposes, it's
important to clone it.

The `#deep_clone` method is defined on `Object` in sexp_processor:

https://github.com/seattlerb/sexp_processor/blob/master/lib/sexp_processor.rb#L613

This addresses issue #27