Adds 'npm run build' before 'npm start' in README.md

[zackees]

What is the purpose of this pull request? (put an "X" next to item)

[x] Documentation update [ ] Bug fix [ ] New feature [ ] Other, please explain:

What changes did you make? (Give an overview)

The readme omitted the required step of npm run build before starting the server.

Which issue (if any) does this pull request address?

It fixes the issue so that users can more easily self host the application.

Is there anything you'd like reviewers to focus on?

No, this should be straight forward.

[welcome[bot]]

🙌 Thanks for opening this pull request! You're awesome.

[welcome[bot]]

🎉 Congrats on getting your first pull request landed!