Finish Heroku upgrade docs

docs/heroku/readme.md

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+# Heroku
+
+- [Upgrade Heroku](/docs/heroku/upgrade.md)

docs/heroku/upgrade.md

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+# Upgrade Cachet on Heroku
+
+To begin, you'll need the [Heroku Toolbelt](https://toolbelt.heroku.com/) on your system.
+
+## Login to Heroku
+
+Run the following command to login to your Heroku account. You'll need to provide your email and password.
+
+```bash
+$ heroku login
+```
+
+## Clone your app
+
+Now clone your app to somewhere on your system, for this example we'll use the desktop and `cachet` as our app name.
+
+```bash
+$ cd ~/Desktop
+$ heroku git:clone -a cachet
+```
+
+The toolbelt will now use Git to clone your app locally.
+
+## Add a new remote
+
+Once cloned we'll want to change into the app directory.
+
+```bash
+$ cd cachet
+```
+
+We can now add a new remote - the Cachet source to the repository.
+
+```bash
+$ git remote add origin git@github.com:cachethq/cachet.git
+```
+
+Now we update our app with the latest version.
+
+```bash
+$ git fetch origin
+$ git reset --hard origin/master
+```
+
+## Push the changes
+
+Our app will now be running the latest version of Cachet, we just need to push it back to Heroku.
+
+```bash
+$ git push heroku
+```
+
+We're done!