Update CLI usage output and readme. (#1225)

This PR improves the CLI usage output. For the commands I figured
they're so simple we can just list them, I moved them to the footer
though so if you type `renterd --help` the commands are the first thing
you see. Also moved the link to docs.sia.tech to the bottom.

```
➜  ./renterd --help

Renterd is the official Sia renter daemon. It provides a REST API for forming
contracts with hosts, uploading data to them and downloading data from them.

There are 3 ways to configure renterd (sorted from lowest to highest precedence):
  - A YAML config file
  - CLI flags
  - Environment variables

Usage:
  -autopilot.accountRefillInterval duration
    	Interval for refilling workers' account balances (default 10s)

...

  -worker.uploadOverdriveTimeout duration
    	Timeout for overdriving slab uploads (default 3s)

There are 3 commands:
  - version: prints the network as well as build information
  - config: builds a YAML config file through a series of prompts
  - seed: generates a new seed and prints the recovery phrase

See the documentation (https://docs.sia.tech/) for more information and examples
on how to configure and use renterd.
```

The `README.md` was extended with a configuration section listing all
config variables and how you can configure them. I moved some stuff
around and removed the `debug` section.


Fixes #859
Fixes #1086