Add documentation for NCCL NET plugins

Also repurpose dummy plugin as example, including headers and
compat layers from v6 to v2.

ext-net/example/Makefile

@@ -0,0 +1,17 @@
+#
+# Copyright (c) 2015-2019, NVIDIA CORPORATION. All rights reserved.
+#
+# See LICENSE.txt for license information
+#
+NCCL_HOME:=../../build/
+CUDA_HOME:=/usr/local/cuda
+INC:= -I$(NCCL_HOME)/include -I$(CUDA_HOME)/include
+PLUGIN_SO:=libnccl-net.so
+
+default: $(PLUGIN_SO)
+
+$(PLUGIN_SO): plugin.c
+	$(CC) $(INC) -fPIC -shared -o $@ -Wl,-soname,$(PLUGIN_SO) $^
+
+clean:
+	rm -f $(PLUGIN_SO)