It seems to be a beta/recent feature on Github, but today I learned we could highlight a blockquote by adding its "types" in the first line:
> **Note**
> This is a note
> **Warning**
> This is a warning
This input will produce the following output:

This might work in every single place as you can write markdown such as READMEs, .md files (that will be rendered in the GitHub platform), comments, etc.