Update README.

README.rst

@@ -3,7 +3,39 @@ Cretonne Code Generator
 =======================
 
 Cretonne is a low-level retargetable code generator. It translates a
-target-independent intermediate language into executable machine code. Cretonne
-aims to generate code that behaves identically on different target
-architectures, and that can be executed safely in a sandbox.
+target-independent intermediate language into executable machine code.
+
+Cretonne is designed to be a code generator for WebAssembly with these design
+goals:
+
+No undefined behavior
+    Cretonne does not have a nasal demons clause, and it won't generate code
+    with unexpected behavior if invariants are broken.
+Portable semantics
+    As far as possible, Cretonne's input language has well-defined semantics
+    that are the same on all target architectures. The semantics are usually
+    the same as WebAssembly's.
+Fast sandbox verification
+    Cretonne's input language has a safe subset for sandboxed code. No advanced
+    analysis is required to verify memory safety as long as only the safe
+    instructions are used. The safe instruction set is expressive enough to
+    implement WebAssembly.
+Scalable performance
+    Cretonne can be configured to generate code as quickly as possible, or it
+    can generate very good code at the cost of slower compile times.
+Predictable performance
+    When optimizing, Cretonne focuses on adapting the target-independent IL to
+    the quirks of the target architecture. There are no advanced optimizations
+    that sometimes work, somtimes fail.
+
+Building the documentation
+--------------------------
+
+To build the Cretonne documentation, you need the `Sphinx documentation
+generator <http://www.sphinx-doc.org/>`_::
+
+    $ pip install sphinx
+    $ cd cretonne/docs
+    $ make html
+    $ open _build/html/index.html