[SM-1259] Add Contributing and cargo fmt Note to README

[coltonhurst]

Type of change

- [ ] Bug fix
- [ ] New feature development
- [ ] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [x] Other

Objective

Adds the contributing note we have in clients & server, with SECURITY.md. Also adds a note about cargo +nightly fmt.

Before you submit

• Please add unit tests where it makes sense to do so