Mention in the README that we avoid using callstack recursion.
Mention in the README that we avoid using callstack recursion.

Callstack recursion has the property that the maximum stack depth can
grow significantly, depending on the input program. Cranelift uses
several recursive algorithms, however it uses explicit heap-based
stacks to do so.

stoklund/cretonne,stoklund/cretonne,stoklund/cretonne