Add a legend to the README.md explaining all the labels meaning.

[johlju]

Details of the scenario you tried and the problem that is occurring:
I think we should add a legend explaining what the labels on issues and PR's mean so it has the same meaning for everybody.
Maybe it's unclear what certain labels mean.

But maybe this one should be added in DscResources repo, and we referencing that here instead.

The DSC configuration that is using the resource (as detailed as possible):
n/a

Version of the Operating System, SQL Server and PowerShell the DSC Target Node is running:
n/a

What module (SqlServer or SQLPS) and which version of the module the DSC Target Node is running:
n/a

Version of the DSC module you're using, or 'dev' if you're using current dev branch:
n/a

[johlju]

This is documented in https://dsccommunity.org/guidelines/maintaining/#labels and in https://github.com/PowerShell/DscResources/blob/master/Automation.md#labels. We should update the list on the DSC Community site.