Cleanup the Metricbeat Developer Guide (#1817)

* Create MetricSet guide is based on generator
* Simplified guide for module files
* Update Metricbeat dev guide with README linking

generate/metricbeat/metricset/README.asciidoc

@@ -0,0 +1,51 @@
+== Create Beat based on Metricbeat
+
+This Metricset beat generator allows to create a beat based on Metricbeat as a library with your
+own Metricsets.
+
+[float]
+=== Requirements
+
+To create your own beat, it is required that Golang 1.6.2 or later are installed, the `$GOPATH` is setup correctly. In
+addition it requires https://www.python.org/downloads/[python] and https://github.com/audreyr/cookiecutter[cookiecutter]
+to be installed. More details on how to install cookiecutter can be found http://cookiecutter.readthedocs.io/en/latest/installation.html[here].
+
+[float]
+=== Step 1 - Clone Beats repository
+
+The first step is to clone the beats repository:
+
+[source,bash]
+----
+git clone https://github.com/elastic/beats $GOPATH/src/github.com/elastic/beats
+----
+
+After having cloned the beat repository into the `GOPATH`, change to the directory where you want to create the beat.
+ This is normally under `$GOPATH/src/github.com/{your-jackfan.us.kg}`. Inside this directory run the command:
+
+
+[float]
+=== Step 2 - Create Beat
+
+[source,bash]
+----
+cookiecutter $GOPATH/src/github.com/elastic/beats/generate/metricbeat/metricset
+----
+
+This will ask you some question about the beat name and path. Complete it accordingly.
+
+
+[float]
+=== Step 3 - Init and Create MetricSet
+
+After having created the beat, change into the beat directory and run:
+
+[source,bash]
+----
+make setup
+----
+
+This will do the initial setup for your beat and also run `make create-metricset`, which will ask you for the
+module name and metricset name of your beat.
+
+For all further details on creating a metricset, check out the https://www.elastic.co/guide/en/beats/metricbeat/current/_create_a_metricset.html[create a MetricSet] docs.