Add Heroku one-click install button

docs/heroku.markdown

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+Deploy Kanboard on Heroku
+=========================
+
+You can try Kanboard for free on [Heroku](https://www.heroku.com/).
+You can use this one click install button or follow the manual instructions below:
+
+[![Deploy](https://www.herokucdn.com/deploy/button.png)](https://heroku.com/deploy?template=https://github.com/fguillot/kanboard)
+
+Requirements
+------------
+
+- Heroku account, you can use a free account
+- Heroku command line tool installed
+
+Manual instructions
+-------------------
+
+```bash
+# Get the last development version
+git clone https://github.com/fguillot/kanboard.git
+cd kanboard
+
+# Push the code to Heroku (You can also use SSH if git over HTTP doesn't work)
+heroku create
+git push heroku master
+
+# Start a new dyno with a Postgresql database
+heroku ps:scale web=1
+heroku addons:add heroku-postgresql:hobby-dev
+
+# Open your browser
+heroku open
+```
+
+Limitations
+-----------
+
+The storage on Heroku is ephemeral, that means uploaded files through Kanboard are not persistent after a reboot.