more: set path guidance

Include information about how to set `PATH`.

Related to racket/racket-lang-org#101

pkgs/racket-doc/scribblings/more/more.scrbl

@@ -95,6 +95,14 @@ start @exec{racket} with no command-line arguments:
   > 
 }
 
+@margin-note{Set your @tt{PATH} environment variable so you can use
+ @exec{raco} and other Racket command line functions. On Mac OS:
+ @tt{sudo sh -c @literal{'}echo "/Applications/Racket v@version{}/bin" 
+ >> /etc/paths.d/racket@literal{'}} (This assumes you have installed
+ Racket in the @tt{Applications} folder). On Windows: add the racket
+ @tt{bin} path to @onscreen{Path} in @onscreen{Environment Variables}
+ (under @onscreen{System Properties}, @onscreen{Advanced} tab)}
+ 
 Assuming that you have Editline installed,@margin-note*{To use GNU
 Readline instead of Editline, set the @envvar{PLT_READLINE_LIB}
 environment variable or install the @filepath{readline-gpl} package.}