Document build process

[TimvdLippe]

This is a task for me. According to the messages in Slack, this build process remains a lot of magic. I should document the build to make it a lot clearer:

1. Why does it exist?
2. How does it work?

[MatthijsKok]

I would also like "Why did we choose this tool for this task (vs other tools) ?"

Some random questions I thought of to try to help you understand what needs clarification:

• Why did we choose bower as our package manager?
• Isn't yarn also a package manager?
• Why do we apparently need both?
• I've been hearing a lot about webpack lately, would that solve any of our problems?
• Can't we just use the polymer CI toolset?

[TimvdLippe]

RE: custom build. Over the past few months the polymer build command in the CLI has significantly improved and should now support most of the use-cases for which we initially had a custom build.