The contribution guide at https://github.com/riscv-software-src/opensbi/blob/master/docs/contributing.md is not up to date

My recent contribution experience leads me to think that the OpenSBI contribution guideline is not accurate. This includes friction to new would-be developers.

I noticed the following issues:

1. https://github.com/riscv-software-src/opensbi/blob/master/docs/contributing.md says that all contributions can be either through emai patches, or through PRs. In reality, only email patches seem to be accepted.
2. There is a protocol by which patches need to have a specific title like [PATCH v3] tag: title, which is never explained in https://github.com/riscv-software-src/opensbi/blob/master/docs/contributing.md The explanation in steps 3 and 4 are not sufficient to formulate a correct patch.
3. A Reviewed-by: tag is required for a merge, which is never explained in https://github.com/riscv-software-src/opensbi/blob/master/docs/contributing.md

For reference, see a "newbie" contributor thread at http://lists.infradead.org/pipermail/opensbi/2023-May/004950.html, and the initial advice given at https://github.com/riscv-software-src/opensbi/pull/304

To be clear, all of the requests are reasonable, but please document them clearly, and ahead of time.

Thanks!

Read docs/contributing.md first, then compare its instructions with the referenced newbie contributor thread and pull request 304. Update the guide to accurately explain the accepted submission route, required patch title format, and Reviewed-by requirement; done means each reported source of contributor friction is documented clearly.