Adding CODES external contributors (non-RPI and non-ANL folks)

CONTRIBUTORS.md

-TODO: add contributors here
+Nikhil Jain (UIUC)
+    - Improvements in credit-based flow control of CODES dragonfly and torus network models.
+    - Addition of direct scheme for setting up dragonfly network topology.
+    - Bug reporter for CODES network models.
+
+Xu Yang (IIT)
+    - Added support for running multiple application workloads with CODES MPI
+      Simulation layer.