Add contributing guidelines

[andreeaflorescu]

Add a document for stating the contributing guidelines. These can also include things like:

• [x] signing your commits (which should nevertheless be tested by the CI)
• [x] following the 50/72 guideline for commit message (also should be enforced by CI)
• [ ] linking the issue that the commit is fixing (so that we can trace back commits to issues & PRs)
• [x] documentation for disclosing security vulnerabilities
• [ ] documentation/testing needed for publishing on crates.io: https://github.com/rust-vmm/community/issues/93

[alxiord]

see also https://github.com/rust-vmm/rust-vmm-ci/issues/14, https://github.com/rust-vmm/rust-vmm-ci/issues/9