include the new library(random) in the overview

Many thanks to @notoria for this contribution!

README.md

@@ -343,6 +343,8 @@ The modules that ship with Scryer Prolog are also called
   for measuring the performance of your code.
 * [`cont`](src/prolog/lib/cont.pl)
   Provides *delimited continuations* via `reset/3` and `shift/1`.
+* [`random`](src/prolog/lib/random.pl)
+  Probabilistic predicates and random number generators.
 
 To read contents of external files, use `phrase_from_file/2` from
 [`library(pio)`](src/prolog/lib/pio.pl) to apply a DCG to