mark multi-form macro expansions with $

A bare list was read as several forms, so a macro returning
`((make-adder 10) 5)' -- a call of what make-adder returns -- was
spliced into two. Add a new $ char to denote splicing, so ($ form ...)
splices.

Readme.org

@@ -212,6 +212,20 @@ Sex has support for syntactic macros. Macro definitions look like
 functions: they have a name, an argument list and a body. Macro should
 return Sex code.
 
+A macro returns *one* form. To return several --- a function beside the
+struct it works on, say --- return them under =$=, which splices them in
+where the macro was written:
+
+#+begin_src scheme
+  (defmacro (pair-of-fns a b)
+    `($ (fn ,a () int (return 1))
+        (fn ,b () int (return 2))))
+#+end_src
+
+=($)= expands to nothing. Everything else is a single form, including
+one whose head is itself a form: =`((make-adder 10) 5)= calls what
+=make-adder= returned, and is not two forms.
+
 *** Examples:
 **** Structure with templated value type
 #+begin_src scheme