Improve docs on get_participants

[Jan-E]

Let us see if the double example passes Travis builds

[codecov-io]

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Merging #14 into master will not change coverage

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[Jan-E]

@andrewheiss I improved the docs on get_participants. See https://github.com/cloudyr/limer/issues/9#issuecomment-258511509 and https://github.com/cloudyr/limer/issues/8#issue-158514544 as well. Please merge.

[andrewheiss]

Thanks!