Add tab warning and future option documentation.

main.py

@@ -33,6 +33,11 @@ Options (* means not implemented):
                        directives like: # 123 "filename".
   optimize             Runs the optimizer.
   optsigns             Optimize signs and float as int.
+  * foldtabs           Tabs can't be copy-pasted, so they aren't optimized by
+                       default. But with support from the viewer, they can be
+                       folded too and make it to the uploaded source. This
+                       option overrides that check, enabling optimization of
+                       strings with tabs.
   * allowcescapes      Enables use of \r, \b, \xNN, \NNN, etc.
   * enableswitch       Enables Firestorm-compatible switch statements
                        (not recommended)