Added release note mention of embedded Python in MSI packages.

docs/markdown/snippets/builtin-python.md

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+# Embedded Python in Windows MSI packages
+
+Meson now ships an internal version of Python in the MSI installer packages.
+This means that it can run Python scripts that are part of your build
+transparently. That is, if you do the following:
+
+    myprog = find_program('myscript.py')
+
+Then Meson will run the script with its internal Python version if necessary.