doc: merge wiki banner changes into README
<!-- PROJECT LOGO -->
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<div align="center">
- <a href="https://github.com/iotaledger/stronghold.rs">
- <img src="https://raw.githubusercontent.com/iotaledger/stronghold.rs/dev/.meta/stronghold_beta.png" alt="Banner">
- </a>
<h3 align="center">Stronghold</h3>
<p align="center">
A Rust library to build a secure software storage for sensitive data.
