Document RAKUDO_OPTIMIZER_DEBUG env var

Author: zoffixznet

doc/Programs/00-running.pod6

@@ -177,4 +177,9 @@ even if there are more tests to be run:
 
     $ PERL6_TEST_DIE_ON_FAIL=1 perl6 t/test-filename.t
 
+=defn C<RAKUDO_OPTIMIZER_DEBUG>, (I<Bool>; C<src/Perl6/Optimizer.nqp>)
+
+When set to a true value, causes to produce extra debugging output, useful for
+debugging Rakudo's static optimizer.
+
 =end pod