configure_file: Add release snippet for encoding keyword

docs/markdown/snippets/configure_file_encoding.md

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+## New encoding keyword for configure_file
+
+Add a new keyword to [`configure_file()`](#Reference-manual.md#configure_file)
+that allows the developer to specify the input and output file encoding.
+
+If the file encoding of the input is not UTF-8 meson can crash (see #1542).
+A crash as with UTF-16 is the best case and the worst meson will silently
+corrupt the output file for example with ISO-2022-JP. For additional details
+see pull request #3135.
+
+The new keyword defaults to UTF-8 and the documentation strongly suggest to
+convert the file to UTF-8 when possible.