Update usage docs

[dantownsend]

The current usage docs are OK for contributors, but this isn't how end users will use it.

piccolo-admin-docker/README.md

Lines 3 to 29 in 376f0ac

	### Usage
	Clone repository.
	```bash
	git clone https://github.com/piccolo-orm/piccolo-admin-docker.git
	```
	Creating an `.env` file.
	```bash
	cp .env.example .env && rm .env.example
	```
	Run the Docker container.
	```bash
	docker-compose up -d
	```
	After site is running log in as admin user on [localhost:8000](http://localhost:8000) and use legacy database.
	Stop the Docker container.
	```bash
	docker-compose down
	```

End users will not clone this repo - they will get the Docker image from Docker Hub, and just run it.

Something like:

docker run piccolo/piccolo_admin