Add explicit step(s) for .env

[allthedoll]

Hey @bkeepers and @JasonEtco 👋

In Configure a GitHub App, step 1 mentions to set WEBHOOK_PROXY_URL in your .env. For people who are newer to .js, it might be helpful to be more explicit here, and mention that there is .env.example that one can use as a template. What do you think?

[bkeepers]

@allthedoll yep, that sounds great! Want to open an PR to update that?

[allthedoll]

PR: probot/probot#414