Add Subzero documentation.

Change-Id: I47fa608e8923605bd090d6216c9ccf7d5c4051e2
Reviewed-on: https://swiftshader-review.googlesource.com/10308
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>
diff --git a/docs/Index.md b/docs/Index.md
index e22f063..12c4318 100644
--- a/docs/Index.md
+++ b/docs/Index.md
@@ -20,7 +20,7 @@
 

 Reactor is an embedded language for C++ to dynamically generate code in a WYSIWYG fashion. It allows to specialize the processing routines for the state and shaders used by each draw call. Its syntax closely resembles C and shading languages, to make the code generation easily readable.

 

-The JIT layer is a run-time compiler, such as [LLVM](http://llvm.org/)'s JIT. Reactor records its operations in an in-memory intermediate form which can be materialized by the JIT into a function which can be called directly.

+The JIT layer is a run-time compiler, such as [LLVM](http://llvm.org/)'s JIT, or [Subzero](Subzero.md). Reactor records its operations in an in-memory intermediate form which can be materialized by the JIT into a function which can be called directly.

 

 Design

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