Document hackathon best practices

[conorsch]

@redshiftzero wrote a great hackathon guide for SD, available as a gist: https://gist.github.com/redshiftzero/81779c42b66738c9478e0f8b830f337f We should fold that into the new site so that we have a URL to share with any and all who are interested.

There's an argument to be made that docs.securedrop.org, i.e. the project docs, would be a better fit, but I'd prefer them on the main website. If you disagree, get loud.

[harrislapiroff]

[Backlog pruning 5/10] This is a docs issue and we're not sure if it's still relevant so Allie will migrate to securedrop-docs for assessment there.