Add start { } to control flow

Author: lizmat

doc/Language/control.pod6

@@ -128,7 +128,33 @@ parenthesize a statement if it is the last thing in an expression:
 =for code :skip-test<syntax error>
 3,    if 1 { 2 }  ; # Syntax error
 
-...which brings us to C<if>.
+=head1 X<start|control flow,start>
+
+The simplest way to run a block B<asynchronously> is by writing C<start>
+before it:
+
+=for code
+start { sleep 1; say "done" }
+say "working";
+# working, done
+
+Note that you need a space between the C<start> and the block.
+
+The C<start {...}> immediately returns a C<Promise> that can be safely ignored
+if you are not interested in the result of the block.  If you B<are> interested
+in the final value of the block, you can call the C<.result> method on the
+returned promise.  So:
+
+    my $promise = start { sleep 10; 42 }
+    # ... do other stuff
+    say "The result is $promise.result()";
+
+If the code inside the block has not finished, the call to C<.result> will
+wait until it is done.
+
+A C<start> may also be used on a bare statement (without curly braces).
+This is mainly just useful when calling a subroutine / method on an object
+is the only thing to do asynchronously.
 
 =head1 X<if|control flow,if>