Update LICENSE.md

[akirataguchi115]

If I am to read this right: https://github.com/core-wg/coap-pubsub/blob/main/CONTRIBUTING.md this repository is licensed under the "simplified BSD license". Do correct me however if I'm wrong. I've already added my comments to the root issue from which this stems from: martinthomson/i-d-template#328

[jaimejim]

Hi Akira, license-wise the same provisions as with other IETF documents apply.
I believe the new repos have the license information below, which is the one I will use:

License

This repository relates to activities in the Internet Engineering Task
Force (IETF). All material in this repository is
considered Contributions to the IETF Standards Process, as defined in
the intellectual property policies of the IETF currently designated as
BCP 78,
BCP 79 and the IETF Trust
Legal Provisions (TLP) Relating to IETF
Documents.

Any edit, commit, pull request, issue, comment or other change made to
this repository constitutes Contributions to the IETF Standards
Process.

You agree to comply with all applicable IETF policies and procedures,
including, BCP 78, 79, the TLP, and the TLP rules regarding code
components (e.g., being subject to a Revised BSD License) in
Contributions.