Add documentation of http interface and add docker run command

[drevicko]

The current documentation on using the module is hard to follow and there is no description of the http interface

[drevicko]

We really need this - we're trying to integrate this module into the platform website, and have not been able to make the docker work.

The content of Vladimir's (@ddwell ) email:

I found it is not exactly trivial to use mixedemotions/but_sentiment.
Do you have any hints/suggestions how to use it?

I have tried to launch the docker image with the following command,
docker run --rm -p 8002:80 -ti mixedemotions/but_sentiment
but I could not get an access to the specified port.