Warning note missing

Ceilometer API has been deprecated, So a warning must be placed to inform about the same and discourage its use.

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [ ] This doc is inaccurate in this way: ______
- [ ] 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: 2.9.1.dev11 on 2017-10-18 22:59
SHA: 3ff6bac3da348bad0b220ab26eab6ad6bfbeb675
Source: https://git.openstack.org/cgit/openstack/python-ceilometerclient/tree/doc/source/index.rst
URL: https://docs.openstack.org/python-ceilometerclient/latest/