Add contribution documentation
A frequent issue delaying the addition of new icons is the lack of documentation on how to contribute the font files to the source tree from an SVG version of the logos (see [#174 (comment)](https://github.com/jpswalsh/academicons/issues/174#issuecomment-1174134346_), [#184 (comment)](https://github.com/jpswalsh/academicons/issues/184#issuecomment-1291144309)).

A simple document explaining the steps to generate the font files (such as a [`CONTRIBUTING`](https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors) file) would streamline the addition of new icons by educating contributors on how to generate the files needed so that they can submit the PR themselves, thus making the process of adding new icons faster, and also allowing the repo maintainers to save the time they would've spent doing it themselves.