add document describing rustfmt's stabilisation process

Processes.md

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+This document outlines processes regarding management of rustfmt.
+
+# Stabilising an Option
+
+In this Section, we describe how to stabilise an option of the rustfmt's configration.
+
+## Conditions
+
+- The design and implementation of the option are sound and clean.
+- The option is well tested, both in unit tests and, optimally, in real usage.
+- There is no open bug about the option that prevents its use.
+
+## Steps
+
+Open a pull request that closes the tracking issue. The tracking issue is listed beside the option in `Configurations.md`.
+
+- Update the `Config` enum marking the option as stable.
+- Update the the `Configuration.md` file marking the option as stable.
+
+## After the stabilisation
+
+The option should remain backward-compatible with previous parameters of the option. For instance, if the option is an enum `enum Foo { Alice, Bob }` and the variant `Foo::Bob` is removed/renamed, existing use of the `Foo::Bob` variant should map to the new logic. Breaking changes can be applied under the condition they are version-gated.