update readme with group example

README.md

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+# GMetric
+
+A pure Ruby client for generating Ganglia 3.1.x+ gmetric meta and metric packets and talking to your gmond / gmetad nodes over UDP protocol. Supports host spoofing, and all the same parameters as the gmetric command line executable.
+
+[http://www.igvita.com/2010/01/28/cluster-monitoring-with-ganglia-ruby/](http://www.igvita.com/2010/01/28/cluster-monitoring-with-ganglia-ruby/)
+
+## Example: Sending a gmetric to a gmond over UDP
+
+```ruby
+Ganglia::GMetric.send("127.0.0.1", 8670, {
+  :name => 'pageviews',
+  :units => 'req/min',
+  :type => 'uint8',
+  :value => 7000,
+  :tmax => 60,
+  :dmax => 300,
+:group => 'test'
+})
+```
+
+## Example: Generating the Meta and Metric packets
+
+```ruby
+g = Ganglia::GMetric.pack(
+  :slope => 'positive',
+  :name => 'ruby',
+  :value => rand(100),
+  :tmax => 60,
+  :units => '',
+  :dmax => 60,
+  :type => 'uint8'
+)
+
+# g[0] = meta packet
+# g[1] = gmetric packet
+
+s = UDPSocket.new
+s.connect("127.0.0.1", 8670)
+s.send g[0], 0
+s.send g[1], 0
+```
+
+### License
+
+The MIT License, Copyright (c) 2009 Ilya Grigorik