Update Readme

[Beasta]

The current instructions don't work for a few reasons. First, the deploy button requires a template in the URL as described here. So it should be something like https://heroku.com/deploy?template=https://github.com/ProjectOpenSea/metadata-api-nodejs/tree/master.

Secondly, heroku git:clone -a your-metadata-api doesn't work on code deployed from the button.

I've updated the Readme to just remove the button in favor the of deployment via the CLI.