Clarify Custom Deployment
Clarify Custom Deployment

This adds:

 * Proper versions of Node required for different Meteor versions.
 * Improved instruction for `MONGO_URL` with link to Mongo docs to explain where it is.  This is the _only_ place where `MONGO_URL` is explained.

 Closes meteor/guide#527

Hansoft/meteor,Hansoft/meteor,kvetoslavnovak/GitBook-Meteor-Guide,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor