refactor cgroup build so non-linux systems see plugin

also updated the README for the fields->files change.

etc/telegraf.conf

@@ -526,6 +526,19 @@
 #   socket_suffix = "asok"
 
 
+# # Read specific statistics per cgroup
+# [[inputs.cgroup]]
+#     ## Directories in which to look for files, globs are supported.
+# 	# paths = [
+# 	#   "/cgroup/memory",
+# 	#   "/cgroup/memory/child1",
+# 	#   "/cgroup/memory/child2/*",
+# 	# ]
+# 	## cgroup stat fields, as file names, globs are supported.
+# 	## these file names are appended to each path from above.
+# 	# files = ["memory.*usage*", "memory.limit_in_bytes"]
+
+
 # # Pull Metric Statistics from Amazon CloudWatch
 # [[inputs.cloudwatch]]
 #   ## Amazon Region