Add docs for Proc::Async.ready (#1315)

Author: the-eater

doc/Type/Proc/Async.pod6

@@ -155,6 +155,14 @@ try sink await $p.start;
 
 Returns C<False> before C<.start> has been called, and C<True> afterwards.
 
+=head2 method ready
+
+    method(Proc::Async:D: --> Promise:D)
+
+Returns a L<Promise|/type/Promise> that will be kept once the process
+has succesfully started. L<Promise|/type/Promise> will be broken if the program
+fails to start.
+
 =head2 method path
 
     method path(Proc::Async:D:)