DESIGN doc, stub out /write endpoint

DESIGN.md

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+# Rojo Design
+This is a super rough draft that I'm trying to use to lay out of my thoughts.
+
+## API
+
+### POST `/read`
+Accepts a `Vec<Route>` of items to read.
+
+Returns `Vec<Option<RbxItem>>`, in the same order as the request.
+
+### POST `/write`
+Accepts a `Vec<{ Route, RbxItem }>` of items to write.
+
+I imagine that the `Name` attribute of the top-level `RbxItem` would be ignored in favor of the route name?
+
+## CLI
+
+### Transform Plugins
+
+## Roblox Studio Plugin