rust: pin-init: README: add information banner on the rename to `pin-init`

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# `pin-init`

> [!NOTE]

> 

> This crate was originally named [`pinned-init`], but the migration to

> `pin-init` is not yet complete. The `legcay` branch contains the current

> version of the `pinned-init` crate & the `main` branch already incorporates

> the rename to `pin-init`.

>

> There are still some changes needed on the kernel side before the migration

> can be completed.

[`pinned-init`]: https://crates.io/crates/pinned-init

<!-- cargo-rdme start -->

Library to safely and fallibly initialize pinned `struct`s using in-place constructors.