Update Heroku button

The button needs a parameter to work, that provides the git repository
that is used for the deployment. This commit corrects the link and this
way fixes the provisioning as it's not working with the wrong/default
buildpacks.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

Author: SISheogorath

README.md

@@ -70,9 +70,11 @@ Thanks for using! :smile:
 
 ## Heroku Deployment
 
-You can quickly setup a sample heroku hackmd application by clicking the button below.
+You can quickly setup a sample Heroku HackMD application by clicking the button below.
 
-[![Deploy](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy)
+[![Deploy on Heroku](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy?template=https://github.com/hackmdio/hackmd/tree/master)
+
+If you deploy it without the button, keep in mind to use the right buildpacks. For details check `app.json`.
 
 ## HackMD by docker container
 [![Try in PWD](https://cdn.rawgit.com/play-with-docker/stacks/cff22438/assets/images/button.png)](http://play-with-docker.com?stack=https://github.com/hackmdio/docker-hackmd/raw/master/docker-compose.yml&stack_name=hackmd)