Move .lintrunner.toml to top-level directory and include basic linting instructions in CONTRIBUTING.md

[vmpuri]

Lintrunner is currently broken since the .lintrunner.toml was moved to the install directory.

This PR updates the necessary paths & moves .lintrunner.toml back to the top-level directory so that lint changes can be applied.

I also provide basic instructions on linter installation & usage in the CONTRIBUTING.md.

Tested using a fresh venv as follows:

python3 -m venv .venv                            
source .venv/bin/activate
./install/install_requirements.sh

pip3 install -r install/requirements-lintrunner.txt

(.venv) puri@puri-mac torchchat % lintrunner -a
Warning: Could not find a lintrunner config at: '.lintrunner.private.toml'. Continuing without using configuration file.
ok No lint issues.
Successfully applied all patches.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchchat/1258

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[Jack-Khuu]

Thanks for doing this

This seems fishy; might be an existing error if you've already moved the file back
Warning: Could not find a lintrunner config at: '.lintrunner.private.toml'. Continuing without using configuration file.