Add high-level to low-level op decompose path

These informative definitions will facilitate progressive lowering
by implementations.

Fix #22
Related #28

charter.html

@@ -220,6 +220,11 @@ <h2>
           macOS/iOS Metal Performance Shaders and Basic Neural Network
           Subroutines.
         </p>
+        <p>
+          For each high-level building block that decomposes into well-known
+          lower-level operations, the APIs will informatively define a generic
+          emulation path to allow for future extensibility.
+        </p>
         <p>It may also work on a higher-level API to load a custom pre-trained Machine Learning model for inference in the browser.</p>
         <section id="section-out-of-scope">
           <h3 id="out-of-scope">