Add roadmap

ROADMAP.md

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+# Bevy Roadmap
+
+* UI Framework
+    * Box model rendering
+    * Use entity hierarchy for tree structure
+    * Styling
+* Rendering
+    * Consider reworking current RenderGraph to be more graph-like
+    * Macro to produce vertex buffer attributes (and maybe descriptors) from structs
+    * Dynamic / user defined shaders
+* Physics
+    * High level physics data types
+    * Integrate with nphysics
+* Editor
+    * Editor <-> game communication protocol
+    * Build UI using bevy UI framework
+    * Consider supporting embedding parts of the editor directly into games