Conform ReadMe and create Contributing Document

[QuintinWillison]

In https://github.com/ably/ably-common/pull/88 we have defined template layouts for README.md and CONTRIBUTING.md, hosted here.

Those documents need conforming in this repository, which necessitates the forking of the current readme content to produce the contributing guide.

This is also a good opportunity to review what's currently here.

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