Indicate use of polling.yaml in documentation "Writing Agent Plugins in ceilometer"

Hi everyone,

I would like to suggest to extend the documentation of "Writing Agent Plugins" regarding the required use of the polling.yaml, where new pollsters most probably have to be listed in order to be polled.

I wrote a Ceilometer pollster plugin, used within the namespace of ceilometer.poll.central. In a previous release it was sufficient to register the pollster in the described entry_points.txt to make it work, because the polling.yaml included all pollsters. After the release of queens, the __init__() of the pollster was executed, but it was not called regularly to poll information. I noticed the pollster has to be put to the polling.yaml, which the documentation does not mention.

*polling.yaml* of my previous environment:
---
sources:
    - name: all_pollsters
      interval: 300
      meters:
        - "*"

*polling.yaml* of my current queens environment:
---
sources:
    - name: some_pollsters
      interval: 300
      meters:
        - cpu
        - cpu_l3_cache
        - memory.usage
        - ...

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- [ ] This doc is inaccurate in this way: ______
- [x] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

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-----------------------------------
Release: 10.0.2.dev2 on 2018-06-26 11:07
SHA: a4aeb8d62dcfad572204f4c2249d6a528f4f44da
Source: https://git.openstack.org/cgit/openstack/ceilometer/tree/doc/source/contributor/plugins.rst
URL: https://docs.openstack.org/ceilometer/queens/contributor/plugins.html