chore: bump actions/checkout to v4, update README, add `.gitattributes`

[kbdharun]

Changes

• CI: Bump actions/checkout from v3 (Node 16) to v4 (Node 20).
• gitattributes: set EOL to LF
• README: update the follow markdown-lint (with the exception being the HTML code, but that's fine)
  • Add spaces between headings and content.
  • Specify code language to code blocks.

[acuteenvy]

Thank you, but may I ask what is the point of shell before the code block? It renders exactly the same:

with ```shell

cargo install tlrc

without ```shell

cargo install tlrc

Is it just a convention?

[kbdharun]

Is it just a convention?

Yeah, we generally follow markdown lint and specify the code block language even if doesn't change rendering.