update contributors

[stevencarlislewalker]

@evanjmitchell is getting ready to submit a manuscript that cites macpan, but this is out of date as of the time of writing: https://mac-theobio.github.io/McMasterPandemic/authors.html#citation

• [ ] @papsti must be an author for goodness sake!
• [ ] @guanwg must also be an author, not a contributor

Open to other suggestions.

[dushoff]

I agree with this, fwiw