Improve and document how to use the distribution as a remote renderer

[xlson]

What would you like to be added:

1. Change the documentation to cover how to use the plugin zip to run the image renderer.
2. Improve the image renderer when running in remote mode from the plugin distribution so that it finds the bundled chrome install by default.

A short guide to running the plugin in remote mode

It's possible to run the plugin in server mode and use it as a remote renderer, just download the latest zip unpack it. Running it requires pointing to the chrome binary.

Create config.json file in plugin folder to point to the chrome binary and possibly add other config (see docs for further details).

{
    "rendering": {
        "chromeBin": "./chrome-linux/chrome-wrapper"
    }
}

Run remote renderer:

./plugin_start_linux_amd64 server --config=config.json

Why is this needed:

I want it to be easy to use the remote renderer, running from source is great for development but not ideal in production. It's also important that our users run a specific release of the image renderer so that we can help debug problems, which is also hard when running from source as it's unlikely that a specific version is being used.