update Heroku deployment guide with webpack

[happysalada]

The current deployment process to heroku won't work with the current phoenix static assets buildpack out of the box. It needs a bit of customization. I was wondering if there is a plan to update the buildpack or if the docs should be updated.

Bascially it's a matter of adding a phoenix_static_buildpack.config file with the following

compile="compile"

and having a compile file (just like a procfile, it's just a file named compile) with the following

yarn deploy
cd ..
mix phx.digest

If the docs need to be updated, I have time to make a PR.

[chrismccord]

It should be updated as part of the 1.4 release. Thanks!

[happysalada]

this was closed by #2834