[docs][PATCH] migration-guide: Add guide for converting licenses

[Joshua Watt <jpewhacker@gmail.com>]

The LICENSE variable is now required to be SPDX license expressions, so
provide a migration guide to describe the changes required to integrate
this change

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
---
 .../migration-guides/migration-6.1.rst        | 58 +++++++++++++++++++
 1 file changed, 58 insertions(+)

diff --git a/documentation/migration-guides/migration-6.1.rst b/documentation/migration-guides/migration-6.1.rst
index 8786cd699..48777e16c 100644
--- a/documentation/migration-guides/migration-6.1.rst
+++ b/documentation/migration-guides/migration-6.1.rst
@@ -75,6 +75,61 @@ And::
 
    oe.utils.any_distro_features("x y", ...) -> bb.utils.contains_any("DISTRO_FEATURES", "x y", ...)
 
+``LICENSE`` is now a SPDX License Expression
+--------------------------------------------
+
+The ``LICENSE`` variable is now an `SPDX License Expression`_ instead of the
+custom license expressions that have been used historically.
+
+The changes required for the new expressions are as follows:
+1. The ``&`` operator is replaced with ``AND``
+
+2. The ``|`` operator is replaced with ``OR``
+
+3. Any license value which is not a valid `SPDX License Expression`_ is an
+   error.
+
+4. Custom (non `SPDX License Identifier`_) licenses are still allowed, as long
+   as they are prefixed with ``LicenseRef-``. The behavior of looking for the
+   license text in ``LICENSE_PATH`` (with or without the ``LicenseRef-``
+   prefix) or ``NO_GENERIC_LICENSE`` is unchanged, and may still be used.
+
+5. ``CLOSED`` as a license is deprecated and will issue a warning. This license
+   is effectively "no license" (usually meaning e.g. "All rights reserved"),
+   but a more precise definition is to provide some sort of actual license text
+   using a custom license. This provides a more consistent definition of the
+   license text, since the meaning of "no license" may vary by jurisdiction.
+   Keep in mind that you can still have a common license file in
+   ``LICENSE_PATH`` and refer to it with a ``LicenseRef-`` license. Note that
+   when you this, you will also need to provide a ``LIC_FILE_CHKSUM`` value to
+   point to your license file.
+
+6. The ``WITH`` operator should now be used to describe an exception to a
+   license, instead of a bespoke license identifier. For example, the old
+   bespoke license ``Apache-2.0-with-LLVM-exception`` would become
+   ``Apache-2.0 WITH LLVM-exception``.
+
+7. Because of the change to use ``WITH`` instead of bespoke licenses, there is
+   a change in how ``INCOMPATIBLE_LICENSE`` works. Anything listed in this
+   variable will match a single licenses (unchanged), but it will also match the
+   left-hand (license) side of a ``WITH`` expression. To allow a license with a
+   specific SPDX license exception, the SPDX license exception must be listed in
+   ``INCOMPATIBLE_LICENSE_EXCEPTIONS``.
+
+   Practically speaking, the place where this comes up the most is when
+   attempting to exclude GPLv3 code using ``INCOMPATIBLE_LICENSE``. Previously,
+   this would have allowed any ``GPLv3-with-exception`` license, since they
+   were bespoke licenses that did not match the ``GPL-3.0* LGPL-3.0*``
+   expansion. Now, however, the licenses will match because they are e.g.
+   ``GPL-3.0-or-later WITH exception``. As such, any exceptions to the GPLv3
+   that should be allowed must be listed in
+   ``INCOMPATIBLE_LICENSE_EXCEPTIONS``.
+
+Currently, the older syntax for license expressions is still parsed and
+automatically converted to an SPDX License Expression, but a warning is issued
+when this occurs. This support will eventually be removed and the only valid
+values for these variables will be SPDX License Expressions.
+
 Removed recipes
 ---------------
 
@@ -93,3 +148,6 @@ Removed classes
 
 Miscellaneous changes
 ---------------------
+
+.. _SPDX License Expression: https://spdx.github.io/spdx-spec/v3.0.1/annexes/spdx-license-expressions/
+.. _SPDX License Identifier: https://spdx.org/licenses/
-- 
2.54.0