Add markdown documentation

Docs/Roadmap and TODO List.md

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+In no particular order:
+
+Secret / semi-secret variables
+
+Secret functions
+
+Global variables
+
+Unicode support for the console
+
+Blockchain integration
+
+Closures instead of a quoted string in menus
+
+GUI that looks for verbs
+
+More sophisticated passage of time
+
+GUI that runs in realtime
+
+The Graphics Engine
+
+Little game demos:
+
+The monopoly game (Errands where you pass thru land owned by others with voronoi diagram)
+
+The Spectra game
+
+Heroes of Rock,Paper,Scissors
+
+Come up with a plan for 'redirect'.
+
+Modify World::scan_near to not allocate any memory, so it can be accessed as a 'getter' in the DrivenEngine.
+
+probes should never modify the state of the world. Currently, there is no protection to guarantee this, except for something in lpxclient where it does a snapshot and rollback before and after the probe. We should come up with a more comprehensive mechanism.
+
+### []()Questionable Design Choices
+
+When we delete a tangible, we currently leave the LUA_TT_TANGIBLE flag on the table, and we leave the metatable in place with the tangible ID in it. Is this the right thing to do? Should we turn it into a truly blank normal table?
+
+### []()Hard Problems without Solutions
+
+**Using Multi-Core CPUs**: We want to do difference transmission in parallel (one thread per connected client). This may be possible because each synchronous model is a separate lua interpreter. However, the master world model is shared. It may be possible to access it in a read-only manner in multiple threads.
+
+**Nonsortable Keys:** what to do about table keys that are other tables, closures, or coroutines?
+
+**Collision Detection**: In the old engine, people could walk through walls. I don't know if that is okay in 2020. But the separation of world-model and graphics engine makes this hard.
+
+**Dealing with Infinite Loops:** if the script goes into an infinite loop, there's no easy way to deal with that. The lua interpreter does have hooks where you theoretically could count cycles.
+
+**Module-Level Locals:** module-level locals are useful to store constants. However, when they're used as variables, they create pseudoglobal state that gets reset every time you edit a source file, and which is unfindable by the difference transmitter.