Set BSD3 license

[jo-mueller]

Title. Sets the license of the spec text both in the myst metadata and in the repo frontpage as a LICENSE file.

[github-actions]

Automated Review URLs

• Readthedocs

[jo-mueller]

@joshmoore should be alright like this? The license is displayed in the repo frontpage like this and appears on the built docs:

[dstansby]

Given much of the content here is derived from https://github.com/ome/ngff, which was made available (at the time it was copied) under https://www.w3.org/copyright/software-license-2015/, the conditions of that license should probably be followed too? In particular, it requires including the following in re-distributions (which this repository effectively is):

• The full text of this NOTICE in a location viewable to users of the redistributed or derivative work.
• Any pre-existing intellectual property disclaimers, notices, or terms and conditions. If none exist, the W3C Software and Document Short Notice should be included.
• Notice of any changes or modifications, through a copyright statement on the new code or document such as "This software or document includes material copied from or derived from [title and URI of the W3C document]. Copyright © [YEAR] W3C® (MIT, ERCIM, Keio, Beihang)."

[jo-mueller]

@dstansby good catch! See #29