README.md

# Description

Adds a security page for indicating process on reporting security
issues.
Also adds a note in readme to point users to the Show & Tell section of
the discussion board to share projects and tutorials.

## Type of change

- This change requires a documentation update

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/

) with
`./isaaclab.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [ ] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

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Signed-off-by: Kelly Guo <kellyguo123@hotmail.com>
Co-authored-by: Mayank Mittal <12863862+Mayankm96@users.noreply.github.com>