Adding description for BuildDFG pass in BuildDFG.cpp

hpvm/lib/Transforms/BuildDFG/BuildDFG.cpp

@@ -6,6 +6,15 @@
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+//
+// BuildDFG pass is responsible for constructing dataflow graph from a textual
+// representation of HPVM IR with HPVM intrinsics from GenHPVM pass. This pass
+// makes use of three crutial abstractions: graph itself, dataflow nodes repre-
+// -senting functions and data edges representing tranfer of data between 
+// the functions (or nodes in the graph). This pass is part of HPVM frontend 
+// and does not make any changes to the textual representation of the IR.
+//
+//===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "buildDFG"
 #include "BuildDFG/BuildDFG.h"