[docs] Update documentation on pull request policy

[nik9000]

After talking to some Elasticsearch folks they mentioned that they now like pull requests to be made up of multiple commits rather then amending the same commit. Before the pull request is merged it'd be squashed but during review it'd keep growing.

It'd be cool to document this on CONTRIBTUING.md and http://www.elasticsearch.org/contributing-to-elasticsearch/ .

If Elasticsearch grows pull request guidelines then can we add: always make the subject of your pull request descriptive of what it does rather then what issue it closes. These subjects turn into email subjects and it is a lot easier to filter what you need to read if the subject is descriptive like "Splort the sort sprocket to make the sort faster" or "Fix overflow in the foo" then "Closes #1234".