chore: use warning labels

[Rexogamer]

Please make sure to check the following tasks before opening and submitting a PR

• [x] I understand and have followed the contribution guide
• [x] I have tested my changes locally and they are working as intended
• [x] These changes do not have any notable side effects on other Revolt projects

This modifies the README to use warning labels instead of the ⚠️ emoji. Here's an example:

[insertish]

oh this is cool

[Rexogamer]

it also supports notes