Add documentation for Deploying to Heroku to README

[lpender]

I think that the process of deploying to Heroku has created enough confusion and frustration that it's time to add a simple summary of the setup required to the documentation about it. This should be updated and expanded or contracted when new issues arise or are resolved.

[StevenXL]

@lpender thank you for submitting this PR. I hope it gets merged soon. It was very helpful for me - i found it a little late but my build kept failing as I had the APPLICATION_HOST set incorrectly.

[composerinteralia]

Ideally we will get this to a place where we can run suspenders my_app, and then deploying to Heroku just works, but having some additional documentation in the meantime makes sense to me. Thanks!

See also #930

[composerinteralia]

Broken Link:

Suggested change

	Read the documentation on [deploying to Heroku](heroku deploy)
	Read the documentation on [deploying to Heroku][heroku deploy]

[composerinteralia]

Now I am getting a 404 for the link. It may be easier to put the text inline into the README

[lpender]

Hey, apologies. I think there are two issues:

1. The link was wrong (didn't contain /blob/master/)
2. The link won't work until this is merged to master

See for reference:

i.e.

Works:

https://github.com/thoughtbot/suspenders/blob/master/README.md

Doesn't Work (yet)

https://github.com/thoughtbot/suspenders/blob/master/docs/heroku_deploy.md

Sorry for the confusion

[composerinteralia]

Thanks!