Fix child workflow already exists and minor README updates

[cretz]

What was changed

• Fixed mistakenly setting child workflow type as run ID on already-exists
  • Also opened Provide already-existing run ID on StartChildWorkflowExecutionFailedEventAttributes if it failed for "already exists" temporal#6961 to get actual run ID here
• Added .NET version info to README
• Clarified workflow-not-instantiable error to mention DI not supported

Checklist

1. Closes [Bug] Child workflow already started puts workflow type where run ID is #371
2. Closes [Feature Request] Clarify support .NET versions/runtimes in README #368
3. Closes [Feature Request] Clarify in workflow constructor exception that DI is not supported #361