[Feature] Deploy example to Heroku

[bdcorps]

Description

This adds a one-click button to deploy GraphQL example on Heroku.

Fixes #9

Type of change

• [x] New feature (non-breaking change which adds functionality)

Notes

Change template URL in ./example/README.md Deploy button from https://heroku.com/deploy?template=https://github.com/bdcorps/graphql-voyager/tree/9-deploy-to-heroku to https://heroku.com/deploy?template=https://github.com/APIs-guru/graphql-voyager/tree/master to deploy the correct application source.