Document licensing requirements for third-party code

[Copilot]

WP-CLI requires all code in the official organization to be MIT-compatible, but this wasn't documented. This creates ambiguity when accepting contributions, bundling code, or adding dependencies.

Changes

• New documentation page (contributions/licensing.md) covering:

  • Compatible licenses (MIT, BSD, Apache 2.0, ISC, CC0, WTFPL)
  • Incompatible licenses (strict GPL-only, copyleft, proprietary, non-commercial)
  • Requirements for bundling third-party code (preserve licenses, maintain copyright notices, clear identification)
  • Dependency license checking best practices (composer show -t for auditing)
  • Clarification on one-way MIT→GPL compatibility

• Updated contributions index to include the new licensing page in handbook navigation

Addresses the concern raised in wp-cli/wp-config-transformer#1 (comment) about lack of documented licensing expectations.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Document licensing expectations</issue_title>
<issue_description>WP-CLI is licensed under the MIT license, and all bundled code should be compatible with that.

We should document our licensing expectations for:

• accepting third-party code into the official repository/organization
• bundling third-party code
• depending on third-party code

Related: wp-cli/wp-config-transformer#1 (comment)</issue_description>

Comments on the Issue (you are @copilot in this section)

@schlessera @miya0001 The MIT license is compatible to the GPL license. As an example, it is also accepted in the plugin repository.

The issue here is about the licensing requirements we have when new package are meant to be added under the official wp-cli organization and/or bundled with WP-CLI.</comment_new>

• Fixes Document licensing expectations #188

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