[Docs] Add VectorizationPlan to docs/Proposals.

Following the request made in https://reviews.llvm.org/D32871, the
general documentation of the Vectorization Plan is hereby placed
under docs/Proposals.

llvm-svn: 304161
diff --git a/llvm/docs/Vectorizers.rst b/llvm/docs/Vectorizers.rst
index a909d45..317271a 100644
--- a/llvm/docs/Vectorizers.rst
+++ b/llvm/docs/Vectorizers.rst
@@ -382,6 +382,17 @@
 
 .. image:: linpack-pc.png
 
+Ongoing Development Directions
+------------------------------
+
+.. toctree::
+   :hidden:
+
+   Proposals/VectorizationPlan
+
+:doc:`Proposals/VectorizationPlan`
+   Modeling the process and upgrading the infrastructure of LLVM's Loop Vectorizer.
+
 .. _slp-vectorizer:
 
 The SLP Vectorizer