Document Regex Interpolation

I think it's worthy to be documented, because people will looking for a way to using patten given at the runtime.

But I'm not sure if this's the only way to interpolate patten(string) into regex.

Author: tisonkun

doc/Language/regexes.pod6

@@ -1255,6 +1255,18 @@ list of predefined subrules is listed in
 L<S05-regex|https://design.perl6.org/S05.html#Predefined_Subrules> of design
 documents.
 
+=head1 X«Regex Interpolation|Regex Interpolation; <$patten>»
+
+If you want to build a regex using pattern given at the runtime, regex
+interpolation is what you looking for.
+
+    sub runtime-regex(Str $patten --> Regex) { /<$patten>/ }
+
+    my $regex = runtime-regex(get);  # INPUT  : «\d+␤»
+    my $text = get;                  # INPUT  : «42␤»
+
+    say $text ~~ $regex;             # OUTPUT : «｢42｣␤»
+
 =head1 Adverbs
 
 Adverbs modify how regexes work and provide convenient shortcuts for