LICENSE.md or LICENSE file missing from repository

[ThisIsMissEm]

Please Indicate One:

• [ ] Editorial
• [ ] Question
• [ ] Feedback
• [x] Blocking Issue
• [x] Non-Blocking Issue

Please Describe the Issue:

I just realised we don't actually have the W3C Document and Software License in this repository, and we should probably rectify that.

Related to: https://github.com/w3c/activitystreams/issues/527

[evanp]

I added a LICENSE.md to the repository using the W3C Software and Documentation license, 2015 version. We'll need to update this to the 2023 version at some point. I'll open a new issue for that.

[ThisIsMissEm]

LGTM

[evanp]

LGTM2