Add docker-compose example to README

[KCErb]

Hi, I'm new to docker-compose and wanted to use this docker container in my project. I'm a little unsure what that would look like. Here's my best guess, can you advise?

php:
  build: ./php/
  links:
    - composer
# skipping lines . . .

composer:
  build: ./composer/
# etc...

And then inside of ./composer/ a Dockerfile like so

FROM composer/composer

# Use local composer.json file
COPY ./composer.json /var/www/html/composer.json
WORKDIR /var/www/html
RUN composer install

Thanks for the great project(s)!

[otravers]

I was struggling with this, here's the comment that solved it for me:
#88 (comment)