Add suggestions how people can contribute

[nojaf]

We should also have a section that lists what people can do to help this project.
Besides PRs for bugfixes, there are some other things that are useful:

• Start formatting a popular open-source project (With --check during CI).
• Contribute to the F# Syntax Tree.
• Contribute to the F# style guide.
• Clean up some of the codebase. (Redudant parentheses for example, update F# Lint).
• Do regression testing on your own projects after each new release.
• Remove old and dead code.
• Improve documentation.
• ...