make dual licensing explicit

[ara4n]

Update readme & LICENSE files to make it explicit that you can buy a commercial license as an AGPL alternative from Element.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.

[erikjohnston]

Curious to see if Github will do the right thing here and display both licenses

[erikjohnston]

Aha, at least it has found the AGPL one. We can probably add a suitable identifier to the commercial license to get it picked up

[erikjohnston]

We can probably add a suitable identifier to the commercial license to get it picked up

This was optimistic of me. There doesn't appear to be any common way of identifying a commercial license