migration-guides: start of 4.3 migration and release notes

From notes from Richard Purdie

(From yocto-docs rev: 62fcaabd06558669034e8732b8d22a9c46e36cf2)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
CC: richard.purdie@linuxfoundation.org
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/migration-guides/migration-4.3.rst

@@ -0,0 +1,73 @@
+.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
+
+Release 4.3 (nanbield)
+========================
+
+Migration notes for 4.3 (nanbield)
+------------------------------------
+
+This section provides migration information for moving to the Yocto
+Project 4.3 Release (codename "nanbield") from the prior release.
+
+.. _migration-4.3-supported-distributions:
+
+Supported distributions
+~~~~~~~~~~~~~~~~~~~~~~~
+
+This release supports running BitBake on new GNU/Linux distributions:
+
+On the other hand, some earlier distributions are no longer supported:
+
+See :ref:`all supported distributions <system-requirements-supported-distros>`.
+
+.. _migration-4.3-go-changes:
+
+Go language changes
+~~~~~~~~~~~~~~~~~~~
+
+-  Support for the Glide package manager has been removed, as ``go mod``
+   has become the standard.
+  
+.. _migration-4.3-recipe-changes:
+
+Recipe changes
+~~~~~~~~~~~~~~
+
+-  Runtime testing of ptest now fails if no test results are returned by
+   any given ptest.
+
+.. _migration-4.3-removed-variables:
+
+Removed variables
+~~~~~~~~~~~~~~~~~
+
+The following variables have been removed:
+
+.. _migration-4.3-removed-recipes:
+
+Removed recipes
+~~~~~~~~~~~~~~~
+
+The following recipes have been removed in this release:
+
+-  ``glide``, as explained in :ref:`migration-4.3-go-changes`.
+
+.. _migration-4.3-removed-classes:
+
+Removed classes
+~~~~~~~~~~~~~~~
+
+The following classes have been removed in this release:
+
+
+.. _migration-4.3-misc-changes:
+
+Miscellaneous changes
+~~~~~~~~~~~~~~~~~~~~~
+
+-  The ``-crosssdk`` suffix and any :term:`MLPREFIX` were removed from
+   ``virtual/XXX`` provider/dependencies where a ``PREFIX`` was used as well,
+   as we don't need both and it made automated dependency rewriting
+   unnecessarily complex. In general this only affects internal toolchain
+   dependencies so isn't end user visible.
+