Addressing README and prettyOncoplot issues

[Kdreval]

This PR is to address the following issues:

• README needs update #57
• prettyOncoplot bugs #58

Pull Request Checklists

Important: When opening a pull request, keep only the applicable checklist and delete all other sections.

Checklist for all PRs

Required

• I ensured not to prepend the functions existing in both GAMBLR.data and GAMBLR.results with <package>::function() syntax.
• I ensured not to add GAMBLR.data or GAMBLR.results to the @import section of the documentation

[Kdreval]

This page of the website is already generated with this branch and shows that both issues in the prettyOncoplot are properly addressed: https://morinlab.github.io/GAMBLR.viz/tutorials/oncoplot.html#annotating-significance-of-mutation-frequencies-in-sample-groups