chore: use markdown rather than HTML in README

[glitched-w0rld]

This PR uses markdown rather HTML in the README file.

[CLAassistant]

All committers have signed the CLA.

[github-actions]

🔍 Build Scan® (commit: 399719d)

Job name	Status	Build Scan®
build-self-hosted-unsafe-jdk-8	✅	https://ge.armeria.dev/s/2p47ljhkubaf2
build-self-hosted-unsafe-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/xit665bjzqsis
build-self-hosted-unsafe-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/jiswu265yey3c
build-self-hosted-unsafe-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/s6fnswygyxvhi
build-self-hosted-unsafe-jdk-17-leak	✅	https://ge.armeria.dev/s/xu5yedep4gusk
build-self-hosted-unsafe-jdk-11	✅	https://ge.armeria.dev/s/viexslyecgfdk
build-macos-12-jdk-21	✅	https://ge.armeria.dev/s/5wpaa6f5kxemu

[ikhoon]

Looks good!

[jrhee17]

👍 👍

[minwoox]

Thanks!