Add compatibility notes to sonar-scanner-cli README / docs

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It is unclear what releases on this image are in active support and which server versions are supported by each of them.

For example, as of now, it appears that both 5.01 and 4.8.1 are both supported (both released nearly simultaneously) but are they? Do they have different sets of SonarQube they are compatible with? etc

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Hi @ericsmalling,

Sorry for the delay to answer you.
We rarely break support between scanner versions and SonarQube so we do not maintain a compatibility chart. Our recommendation is to please use the latest scanner version.

The docker image is similar. We recommend using the latest fixed tag version. The docker image has its own release lifecycle, so the main versions may differ, but under the hood, it wraps the latest scanner version.
Please let me know if you have further questions or if you run into any problems.