[docs] Add initial Global ISel documentation.

This reflects the current state of Global ISel.  As progress is
made, we'll document our design decisions in it.
Comments very welcome!

llvm-svn: 286002

llvm/docs/index.rst

@@ -272,6 +272,7 @@ For API clients and LLVM developers.
    FaultMaps
    MIRLangRef
    Coroutines
+   GlobalISel
 
 :doc:`WritingAnLLVMPass`
    Information on how to write LLVM transformations and analyses.
@@ -390,6 +391,9 @@ For API clients and LLVM developers.
 :doc:`Coroutines`
   LLVM support for coroutines.
 
+:doc:`GlobalISel`
+  This describes the prototype instruction selection replacement, GlobalISel.
+
 Development Process Documentation
 =================================