Add a Proof Of Concept doc

docs/poc.md

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+Proof Of Concept
+================
+
+For a proof of concept we'll implement a very small subset of the final features
+
+### Core Features
+
+* Create an account
+* Control an entity in a 2D map
+* Query for what the entity can see
+* Persistent accounts and world
+* Multiple users
+
+### Stretch goals
+
+* Rover inventory
+* Rover internals
+* Basic time mechanics
+* Basic survival mechanics
+
+### Key missing features
+
+* No token security (very hackable)
+* No time dilation effects
+* No global coordinates
+* No render of rover camera view
+* No rover replacement mechanic
+* No advanced app/interface