Add information to README about Docker image & Docker Hub

[flaksp]

This pull request relates to #657.

[coveralls]

Coverage remained the same at 82.006% when pulling 1776754 on neluzhin:patch-1 into a665254 on Rebilly:master.

[RomanHotsiy]

Nice! The one issue is that the badge tells "Repo not found":

But I think that may be caching issue.

Thanks for your efforts!

[RomanHotsiy]

Also, do you know how to force Docker Hub to not overwrite README.io each time or use a separate one?

[flaksp]

@RomanGotsiy, seems like you can't use your description, because it will be overwritten after each build. As described in docs:

The build process looks for a README.md in the same directory as your Dockerfile. If you have a README.md file in your repository, it is used in the repository as the full description. If you change the full description after a build, it’s overwritten the next time the Automated Build runs. To make changes, modify the README.md in your Git repository.

[flaksp]

Nice! The one issue is that the badge tells "Repo not found":

@RomanGotsiy, I see you've created "Repository", not "Automated Build", that's because badge can't find your repo:

Am I right?