[PDB] Begin adding documentation for the PDB file format.

Differential Revision: https://reviews.llvm.org/D26374

llvm-svn: 286491
diff --git a/llvm/docs/PDB/HashStream.rst b/llvm/docs/PDB/HashStream.rst
new file mode 100644
index 0000000..a758db4
--- /dev/null
+++ b/llvm/docs/PDB/HashStream.rst
@@ -0,0 +1,3 @@
+=====================================

+The TPI & IPI Hash Streams

+=====================================