chore: update CONTRIBUTING.md for new process (#3366)

- matches standard Unicode text

CONTRIBUTING.md

@@ -204,8 +204,17 @@ The following PR has one non-blocking review, one blocking review, one approval,
 
 ## Licenses
 
-See the file called [LICENSE](LICENSE) for terms applying to your contribution.
-When creating the PR, you will be asked to [sign the CLA](https://cla-assistant.io/unicode-org/icu4x).
+### Contributor License Agreement
+
+In order to contribute to this project, the Unicode Consortium must have on file a Contributor License Agreement (CLA) covering your contributions, either an individual or a corporate CLA. Pull Requests will not be merged until the correct CLA is signed. Which version needs to be signed depends on who owns the contribution being made: you as the individual making the contribution or your employer. _It is your responsibility to determine whether your contribution is owned by your employer._ Please review [The Unicode Consortium Intellectual Property, Licensing, and Technical Contribution Policies][policies] for further guidance on which CLA to sign, as well as other information and guidelines regarding the Consortium’s licensing and technical contribution policies and procedures.
+
+- **Individual CLA**: If you have determined that the Individual CLA is appropriate, just open a Pull Request and you will have the opportunity to click to accept the Individual CLA.
+
+- **Corporate CLA**: If you have determined that a Corporate CLA is appropriate, please check the [public list of Corporate CLAs][unicode-corporate-clas] that the Consortium has on file. If your employer has already signed a CLA, then just open a Pull Request and you will have the opportunity to click that your employer has already signed a CLA. If your employer has not already signed a CLA, you will need to arrange for your employer to sign the Corporate CLA, as described in [How to Sign a Unicode CLA][signing].
+
+Unless otherwise noted in the LICENSE file, this project is released under the free and open-source [Unicode License][unicode-license], also known as Unicode, Inc. License Agreement - Data Files and Software.
+
+SPDX-License-Identifier: Unicode-DFS-2016
 
 ### New files
 
@@ -277,3 +286,7 @@ _(followed by the original boilerplate from Unicode data)_
 Please discuss first.
 
 [style_guide]: docs/process/style_guide.md
+[policies]: https://www.unicode.org/policies/licensing_policy.html
+[unicode-corporate-clas]: https://www.unicode.org/policies/corporate-cla-list/
+[signing]: https://www.unicode.org/policies/licensing_policy.html#signing
+[unicode-license]: https://www.unicode.org/license.txt