[doc] How to use the pandoc/extra docker image (#319)

* Add documentation in the README on how to use the `pandoc/extra` docker image

README.md

@@ -23,6 +23,36 @@ A clean **pandoc LaTeX template** to convert your markdown files to PDF or LaTeX
 
     If there are no folders called `templates` or `pandoc` you need to create them and put the template `eisvogel.latex` inside. You can find the default user data directory on your system by looking at the output of `pandoc --version`.
 
+
+### Docker image
+
+Alternatively, if you don't want to install LaTeX, you can use the Docker 
+image named [pandoc/extra]. The image contains pandoc, LaTeX and a curated
+selection of components such as the eisvogel template, pandoc filters and
+open source fonts. A common use of the image looks like this 
+(line breaks for readability):
+
+``` bash
+docker run --rm \
+       --volume "$(pwd):/data" \
+       --user $(id -u):$(id -g) \
+       pandoc/extra example.md -o example.pdf --template eisvogel --listings
+```
+
+For frequent command line use, you can define the following shell alias:
+
+``` bash
+alias pandock='docker run --rm -v "$(pwd):/data" -u $(id -u):$(id -g) pandoc/extra'
+```
+
+The example invocation with Docker from above now looks like this:
+
+``` bash
+pandock example.md -o example.pdf --template eisvogel --listings
+```
+
+[pandoc/extra]: https://hub.docker.com/r/pandoc/extra
+
 ## Usage
 
 1.  Open the terminal and navigate to the folder where your markdown file is located.