Document PERL6_TEST_TIMES env var

Fixes #738

Author: zoffixznet

doc/Programs/00-running.pod6

@@ -132,4 +132,27 @@ C<IO::Spec::Cygwin> inherits this from C<IO::Spec::Unix>.
 C<IO::Spec::Win32.path> will read the first defined of either C<%PATH%> or C<%Path%> as a
 semicolon-delimited list.
 
+=defn C<PERL6_TEST_TIMES>, (I<Bool>; C<lib/Test.pm6>)
+
+When set to a true value will cause timing information to be printed when
+the test suite is running:
+
+    $ PERL6_TEST_TIMES=1 perl6 -MTest -e 'plan 2; is sleep(2), Nil; nok sleep(.5)'
+    1..2
+    # between two timestamps 1 microseconds
+    ok 1 -
+    # t=2000562
+    ok 2 -
+    # t=500388
+
+The C<# between two timestamps> is printed when C<plan> is called and
+indicates how long it takes
+just to read the clock to produce a timestamp. C<# t=> values that follow
+are printed after each test function is executed and the value represents
+the time in microseconds that elapsed from the previous timestamp (or the
+C<# between...> message).
+
+Note that if you use C<done-testing> instead of C<plan>, the C<# between...>
+message will not be displayed and the first C<# t=> value will be a C<NaN>.
+
 =end pod