Draft for the Heroku deployment (not working yet)

Hi,

I drafted a 2-repo deployment for Heroku. However, it does not work yet, I think that the `METEOR_PACKAGE_DIRS` is incorrect though I can't fix it, as [described in this issue](https://github.com/AdmitHub/meteor-buildpack-horse/issues/188](https://github.com/AdmitHub/meteor-buildpack-horse/issues/188
)

Also, I have no idea how you would handle a private repo for the custom Vulcan packages. However most of the time you'll use this kind of install either to enjoy the latest PR before they are merged into Vulcan or on the opposite to keep old versions of the packages, so I guess very few people will need to hide their packages code in a second private repo, they'll use the usual install process instead and put those private files directly into their app code.

source/deployment.md

@@ -252,6 +252,42 @@ To restart your App, type
 To delete your App, type
 `pm2 delete myappname`
 
+## Heroku with a Two-Repo install
+
+**Note: if you did not follow the optional [two-repo install](http://docs.vulcanjs.org/index.html#Two-Repo-Install-Optional), then simply follow the usual Meteor app deployment process as [described here](https://www.coshx.com/blog/2016/08/19/how-to-deploy-a-meteor-1-4-app-to-heroku/).**
+
+### Deloy the package repo
+
+Host your custom Vulcan packages repo on a public registry such as GitHub.
+
+In your application, add the packages repo as a submodule of your app repo, **in a hidden folder**. Hidden folder are ignored by your Meteor app. Otherwise it would eagerly load all modules from the packages repo, which you don't want. Actually any ignored folder would be ok (`imports` etc.). Name is arbitrary too, though `.submodules` is clear.
+
+```
+git submodule add https://your-packages-public-repo .submodules/your-packages-repo-name
+```
+
+Then, add a script to install the npm packages of this submodule every time Heroku triggers a build:
+
+```
+heroku-postbuild: "cd .submodules/my-repo-name && npm i"
+```
+
+Finally, in your settings or env variables set:
+```
+METEOR_PACKAGE_DIRS="/app/.submodules/my-repo-name/packages"
+```
+
+`/app` is your app folder in Heroku, exactly the same as your app folder on your computer.
+
+
+### Update the package repo
+
+If you need to update the submodule, simply go into its folder and pull:
+```
+cd .submodules/my-repo-name && git pull
+```
+You can then commit. If Heroku is unhappy (e.g if you deleted a commit), delete the submodule completely, push on Heroku, and readd it.
+
 ### References
 
 - [How to deploy Meteor Apps with pm2-meteor](http://pm2-meteor.betawerk.co/)