Update `All Contributors`

[joshuajames-smith]

The repo's README.md Contribution section needs updated to reflect contributions. Complete via All Contribution Bot.

[joshuajames-smith]

@all-contributors please add @philhallbio for documentation, maintenance and content

[allcontributors]

@joshuajames-smith

I've put up a pull request to add @philhallbio! 🎉