Add Heroku deployment instructions with Recline git submodule

[augusto-herrmann]

The application is set up for automatic deployment to Heroku, but there is a dependency on Recline (now datopian/portal.js) in the form of a Git submodule (think of a git repo inside another git repo).

There were supposed to be Heroku deployment instructions in the README, but it now points to a broken link and the text would probably be outdated anyway.

We need either to document how to deploy properly to Heroku using the git submodule or to figure out a way to use Recline/Portal.js as a package dependency instead of a git submodule. Or else the site will probably keep breaking again regularly (#200).