[README.md] Adding Docker instructions for local building

[ar3s3ru]

Jekyll requires different stuff in order to be used (e.g. Ruby and Gems).
For those (like me :D) who don't like to install the whole Jekyll package and its dependencies, a Docker one-shot ephemeral container works just fine for local building.

To build using jekyll/jekyll Docker image, use this command:

docker run --rm -it \
-p 4000:4000 \
--volume "/absolute/path/to/site/repo":/srv/jekyll \
jekyll/jekyll jekyll serve

Adding this method in the README.md would be great.