Add release-notes snippet for install_umask

docs/markdown/snippets/install_umask.md

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+## New built-in option install_umask with a default value 022
+
+This umask is used to define the default permissions of files and directories
+created in the install tree. Files will preserve their executable mode, but the
+exact permissions will obey the install_umask.
+
+The install_umask can be overridden in the meson command-line:
+
+    $ meson --install-umask=027 builddir/
+
+A project can also override the default in the project() call:
+
+    project('myproject', 'c',
+      default_options : ['install_umask=027'])
+
+To disable the install_umask, set it to 'preserve', in which case permissions
+are copied from the files in their origin.