Add REPL to the roadmap

doc/roadmap.rst

@@ -43,6 +43,21 @@ play any Pygame Zero game, in order to follow the principle of
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+REPL
+----
+
+Python's REPL is a valuable feature that makes the language much more
+accessible to beginners.
+
+Python programs that present a user interface, however, rarely provide a REPL,
+because the main thread is used to run the event loop for the user interface.
+
+However, Javascript in a web browser demonstrates how this can work. You have
+access to an interactive REPL that allows investigating program state *while
+you interact with the interface*. This is very useful for debugging, and would
+be a powerful addition to Pygame Zero.
+
+
 Surface juggling
 ----------------