chore: Add shields to README.adoc

[stoobie]

Description of the Changes

The title says it all.

PR Preview URL

https://github.com/starknet-io/starknet-docs/tree/steve/add-shields-to-README

Check List

• [ ] Changes have been done against main branch, and PR does not conflict
• [ ] PR title follows the convention: <docs/feat/fix/chore>(optional scope): <description>, e.g: fix: minor typos in code

---

This change is 

[starknet-bot]

Your preview build is ready! ✨ Check the following link in 1-2 minutes: https://starknet-io.github.io/starknet-docs/pr-1254/documentation/ .

[starknet-bot]

Your preview build is ready! ✨ Check the following link in 1-2 minutes: https://starknet-io.github.io/starknet-docs/pr-1254/documentation/ .