Merge pull request #2493 from dhodder/licensing_doc

CONTRIBUTING: Add licensing details (#1994)

CONTRIBUTING.md

@@ -24,6 +24,8 @@ search the issues to make sure no one else is working on it.
 1. The code must be clippy-warning-free and rustfmt-compliant.
 1. Don't hesitate to move common functions into uucore if they can be reused by other binaries.
 1. Unsafe code should be documented with Safety comments.
+1. uutils is original code. It cannot contain code from existing GNU or Unix-like
+   utilities, nor should it link to or reference GNU libraries.
 
 ## Commit messages
 
@@ -82,3 +84,29 @@ uutils: add new utility
 ```
 gitignore: add temporary files
 ```
+
+## Licensing
+
+uutils is distributed under the terms of the MIT License; see the `LICENSE` file
+for details. This is a permissive license, which allows the software to be used
+with few restrictions.
+
+Copyrights in the uutils project are retained by their contributors, and no
+copyright assignment is required to contribute.
+
+If you wish to add or change dependencies as part of a contribution to the
+project, a tool like `cargo-license` can be used to show their license details.
+The following types of license are acceptable:
+
+* MIT License
+* Dual- or tri-license with an MIT License option ("Apache-2.0 or MIT" is a popular combination)
+* "MIT equivalent" license (2-clause BSD, 3-clause BSD, ISC)
+* License less restrictive than the MIT License (CC0 1.0 Universal)
+
+Licenses we will not use:
+
+* An ambiguous license, or no license
+* Strongly reciprocal licenses (GNU GPL, GNU LGPL)
+
+If you wish to add a reference but it doesn't meet these requirements, please
+raise an issue to describe the dependency.