Add README and TODO.

README.md

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+Sibyl
+=====
+
+This is a parser and interpreter for smart answers that aims to address a few
+challenges with our current embedded Ruby implementations, specifically:
+
+* Verifying correctness: no cycles, dead ends, or unreachable nodes
+* Retaining readability and terseness
+
+It's named after the prophets of antiquity. Why Sibyl? Well, I could hardly
+call it Oracle or Delphi, could I?
+
+Syntax
+------
+
+The syntax is relatively simple, and consists of metadata, steps, and outcomes.
+The grammar is specified in the file `lib/sibyl/parser.rb`; some examples
+follow:
+
+    -- Metadata
+    metadata need 1660
+    metadata status published
+
+    -- Define a multiple-choice step
+    step multiple "step a"
+      option foo -> "step b"
+      option bar -> "step c"
+
+    -- Define a direct transition that sets a variable
+    step number "step c"
+      set x { input }
+      go -> "step d"
+
+    -- Choose a step based on a calculation
+    step number "step d"
+      set y { input }
+      go ->
+        if { y > x } -> "step e"
+        otherwise    -> "step f"
+
+    -- or
+    step multiple "step f"
+      option baz ->
+        if { something? } -> "step g"
+        if { something_else? } -> "step h"
+        otherwise -> "step i"
+      option quuz -> "step j"
+
+    -- Define final steps
+    outcome "step j"
+
+Usage
+-----
+
+    require "sibyl/graph"
+    graph = Sibyl::Graph.new(source)
+    graph.validate! # raises an exception if the graph is incorrect
+    step = graph.walk(["yes", "a", "1"])

TODO.txt

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+* Create input classes for input types to deserialise numbers, dates, etc.
+* Expose expected input for use by web side.