Add documentation for configuring via ENV Vars

[24367dfa]

Here i saw that yopass loads configuration from the environment.

I tested it with a simple docker-compose file and would like to add some documentation to the README.

services:
...
  yopass:
    image: jhaals/yopass
    restart: always
    ports:
      - "127.0.0.1:8080:8080"
    environment:
      - MAX_LENGTH=1000
      - PORT=8080
      - MEMCACHED=memcached:11211
...

PS: viper has a function to define an application specific env-var prefix, that might be an option to avoid name conflicts.