[matthew-iabtechlab]

Adds contribution terms drafted by outside counsel so that contributions from non-members come with a patent license (or non-assert) as well as a copyright license. Adds CONTRIBUTING.md containing just the IPR section, adds a PR template checkbox that has contributors accept the terms, and points the README's contribution section at CONTRIBUTING.md. Members remain under the IAB Tech Lab IPR Policy.

Companion to IABTechLab/OpenProposal#136.

Adapted from the OpenProposal text (this repo is the AAMP hub, not a spec repo):

• The hub is licensed under Apache 2.0 (no CC BY 3.0 spec, no scripts/ or LICENSE-CODE), so paragraph (a) says Apache License 2.0 and the CC BY / code-licence sentences are dropped.
• "OpenProposal Specification" is replaced with "AAMP framework", the term the README uses.
• The README had no existing submission/IPR paragraph, so one bullet is added under "Governance & Contribution Model".
• The PR template keeps the "What this changes" and "Contribution terms" sections; the spec-field checklist items don't apply to this repo.
• Counsel should confirm this Apache-only wording.

Open item before merging: confirm the contact email in CONTRIBUTING.md (draft uses support@iabtechlab.com). Shailley to confirm.

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