Update heroku instructions

docs/Running-Mastodon/Heroku-guide.md

@@ -3,13 +3,52 @@ Heroku guide
 
 [![Deploy](https://www.herokucdn.com/deploy/button.svg)](https://dashboard.heroku.com/new?button-url=https://github.com/tootsuite/mastodon&template=https://github.com/tootsuite/mastodon)
 
-Mastodon can theoretically run indefinitely on a free [Heroku](https://heroku.com) app. It should be noted this has limited testing and could have unpredictable results.
+Mastodon can be run on a free [Heroku](https://heroku.com) app. It should be
+noted this has limited testing and could have unpredictable results.
 
-1. Click the above button.
-2. Fill in the options requested.
-  * You can use a .herokuapp.com domain, which will be simple to set up, or you can use a custom domain. If you want a custom domain and HTTPS, you will need to upgrade to a paid plan (to use Heroku's SSL features), or set up [CloudFlare](https://cloudflare.com) who offer free "Flexible SSL" (note: CloudFlare have some undefined limits on WebSockets. So far, no one has reported hitting concurrent connection limits).
-  * You will want Amazon S3 for file storage. The only exception is for development purposes, where you may not care if files are not saved. Follow a guide online for creating a free Amazon S3 bucket and Access Key, then enter the details.
-  * If you want your Mastodon to be able to send emails, configure SMTP settings here (or later). Consider using [Mailgun](https://mailgun.com) or similar, who offer free plans that should suit your interests.
-3. Deploy! The app should be set up, with a working web interface and database. You can change settings and manage versions from the Heroku dashboard.
+## Basic setup
 
-You may need to use the `heroku` CLI application to run `USERNAME=yourUsername rails mastodon:make_admin` to make yourself an admin.
+Click the button above to start creating a Heroku app with the Mastodon repo as
+the source. This tells Heroku to use the `app.json` file which does things like
+prompt for config variables, set up the right buildpacks, run a postdeploy task,
+and add the appropriate addons.
+
+If you don't use the deploy button and app.json approach, you will need to do
+some of that manually.
+
+## Domain names and SSL
+
+You can add your domain name to the Heroku app's setting, and then also use
+Heroku's (free) auto renewal program for Lets Encrypt certificates, by
+requesting a cert from the settings screen. You'll have to point your hostname
+DNS at Heroku using the values heroku gives you on this screen, using whatever
+method is appropriate for your DNS setup.
+
+You should set the Heroku config vars of `LOCAL_DOMAIN` to your hostname, and
+`LOCAL_HTTPS` to "true" as well.
+
+## Email
+
+Consider using [Mailgun](https://mailgun.com) or similar, who offer free plans
+that should suit your interests. Look in `production.rb` to see which config
+variables need to be set on Heroku for outgoing email to work.
+
+## File storage
+
+You will want Amazon S3 for file storage. The only exception is for development
+purposes, where you may not care if files are not saved. Follow a guide online
+for creating a free Amazon S3 bucket and Access Key, then enter the details.
+
+## Deployment
+
+You can deploy from the Heroku web interface or from the command line. Run:
+
+  `heroku run rails db:migrate`
+
+after you first deploy to set up the first database.
+
+You may need to use the `heroku` CLI application to run:
+
+  `USERNAME=yourUsername rails mastodon:make_admin`
+
+to make yourself an admin.