Add release note snippet for augments.

docs/markdown/snippets/optionrefactor.md

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+## Per project subproject options rewrite
+
+A common requirement when building large projects with many
+subprojects is to build some (or all) subprojects with project options
+that are different from the "main project". This has been sort of
+possible in a limited way but is now possible in a general way. These
+additions can be added, changed and removed at runtime using the
+command line or, in other words, without editing existing
+`meson.build` files.
+
+Since this release you can specify _augments_, which are basically
+per-project option settings. These can be specified for every top
+level (i.e. not project) options. Suppose you have a project that has
+a single subproject called `numbercruncher` that does heavy
+computation. During development you want to build that subproject with
+optimizations enabled but your main project without
+optimizations. This can be done by specifying an augment to the given
+subproject:
+
+    meson configure -Anumbercruncher:optimization=3
+
+Another case might be that you want to build with errors as warnings,
+but some subproject does not support it. It would be set up like this:
+
+    meson configure -Dwerror=true -Anaughty:werror=false
+
+You can also specify an augment on the opt level project. A more
+general version of enabling optimizations on all subprojects but not
+the top project would be done like this:
+
+    meson configure -Doptimization=2 -A:optimization=0
+
+Note the colon after `A`.
+
+Augments can be removed with -U
+
+    meson configure -Usubproject:optionnname
+
+This is a major change in how options are handled. Current
+per-subproject options are converted to augments on the fly. It is
+expected that the logic might be changed in the next few releases as
+logical gaffes are discovered.
+
+We have tried to keep backwards compatibility as much as possible, but
+this may lead to some build breakage.