Improve documentation

[kvrhdn]

The README is getting a bit long and the action is aggregating quite a bit of logic that is worth documenting.

We could split up the documentation in information about:

• using the action: the syntax, the available inputs and outputs
• its behavior: the trace ID, how to uses buildevents internally, how to add your own fields, how to connect spans
• development: how to contribute, how to build, release procedure

To consider:

• I believe the README is shown in the sidebar when editing GitHub Actions workflows

TODO

• Document how to override BUILDEVENT_FILE and add your own arbitrary fields (we might be able to refer to the buildevents docs?) #3 (comment), #18
• Document meta.source field #19