Add information about SpiderMonkey and rustc plans.

spidermonkey.rst

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+========================
+Cretonne in SpiderMonkey
+========================
+
+`SpiderMonkey <https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey>`_ is the
+JavaScript and WebAssembly engine in Firefox. Cretonne is designed to be used in SpiderMonkey with
+the goal of enabling better code generation for ARM's 32-bit and 64-bit architectures, and building
+a framework for improved low-level code optimizations in the future.
+
+Phase 1: WebAssembly
+--------------------
+
+SpiderMonkey currently has two WebAssembly compilers: The tier 1 baseline compiler (not shown
+below) and the tier 2 compiler using the IonMonkey JavaScript compiler's optimizations and register
+allocation.
+
+.. image:: media/spidermonkey1.png
+    :align: center
+    :width: 80%
+    :alt: Cretonne in SpiderMonkey phase 1
+
+In phase 1, Cretonne aims to replace the IonMonkey-based tier 2 compiler for WebAssembly only. It
+will still be orchestrated by the BaldrMonkey engine and compile WebAssembly modules on multiple
+threads. Cretonne translates binary wasm functions directly into its own intermediate
+representation, and it generates binary machine code without depending on SpiderMonkey's macro
+assembler.
+
+Phase 2: IonMonkey
+------------------
+
+The IonMonkey JIT compiler is designed to compile JavaScript code. It uses two separate
+intermediate representations to do that:
+
+- MIR is used for optimizations that are specific to JavaScript JIT compilation. It has good
+  support for JS types and the special tricks needed to make JS fast.
+- LIR is used for register allocation.
+
+.. image:: media/spidermonkey2.png
+    :align: center
+    :width: 80%
+    :alt: Cretonne in SpiderMonkey phase 2
+
+Cretonne has its own register allocator, so the LIR representation can be skipped when using
+Cretonne as a backend for IonMonkey.