Document TraceId and SpanId in ReadMe

Is your feature request related to a problem? Please describe.
No, just requesting https://github.com/serilog/serilog-expressions/blob/dev/README.md does include documentation for TraceId and SpanId which seems to be working based on an example I discovered here earlier.

Describe the solution you'd like
Update https://github.com/serilog/serilog-expressions/blob/dev/README.md#properties to include documentation for TraceId and SpanId.

Describe alternatives you've considered
N/A

Additional context
N/A

PR #113