ref-manual: add LTS and Mixin terms

(From yocto-docs rev: f9ce60e2a035f3921901d2c6633df6e302cad1c7)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/terms.rst

@@ -211,6 +211,12 @@ universal, the list includes them just in case:
       Layers`" section in the Yocto Project Board Support Packages (BSP)
       Developer's Guide.
 
+   :term:`LTS`
+      This term means "Long Term Support", and in the context of the Yocto
+      Project, it corresponds to selected stable releases for which bug and
+      security fixes are provided for at least four years. See
+      the :ref:`ref-long-term-support-releases` section for details.
+
    :term:`Metadata`
       A key element of the Yocto Project is the Metadata that
       is used to construct a Linux distribution and is contained in the
@@ -230,6 +236,12 @@ universal, the list includes them just in case:
       :yocto_git:`yocto-kernel-cache </yocto-kernel-cache>`
       Git repository.
 
+   :term:`Mixin`
+      A :term:`Mixin` layer is a layer which can be created by the community to
+      add a specific feature or support a new version of some package for an
+      :term:`LTS` release. See the :ref:`ref-long-term-support-releases`
+      section for details.
+
    :term:`OpenEmbedded-Core (OE-Core)`
       OE-Core is metadata comprised of
       foundational recipes, classes, and associated files that are meant to