Come up with and document policy for adding new event kinds

[michaelwoerister]

Right now we have a few event kinds, basically the ones defined in https://github.com/rust-lang/measureme/blob/e6f55c7bcacc1c70b895c858d1840958709b9a34/measureme/src/rustc.rs.

If there is an event that is not recognized some tools will print out a warnings (e.g. summarize). Others might just ignore them.

I think we should document somewhere:

• How to add new event kinds
• What kind of support is expected for each kind of event (e.g. that all tools should be able to handle all known event kinds gracefully)
• How to get approval for adding new events

I think that in general that it should be easy to add new event kinds (implementation-wise) but there should be a thorough review if it's really necessary to have a new event kind or if an existing event kind can be used instead.