relax rate requirements
spell out that we're ignoring velocity for now

* Various tweaks following initial implementation:
- switched from `m.beacon.*` state event to `m.becon` reference relationship to original `m.beacon_info.*`
- changed `beacon_info` created to `m.ts`
- renamed `lifetime` to `timeout`
- added `m.asset` `type`

* Clarify that this proposal includes history - see MSC3672 for EDUs
* Note dependency on MSC3267 for reference relationships
* Remove variable event_type - instead use state_key
* Propose state_key prefixes inspired by MSC3757 and MSC3779
* Update links to matrix-spec-proposals
* Clarify how to stop sharing, and when a share is stopped
* Mention the need for redactions to work
* Clarify the use of milliseconds for all timing
* Mention implementations