Update readme. Include BeautifulSoup requirement and usage
added plantuml_rst plugin

Added a plugin that defines a 'uml' directive to be able to use PlantUML
diagrams with rst documents.
Merge pull request #212 from jkrshw/bugfix/rep-image-readme

[representative_image] Update README, including BeautifulSoup requirement and usage.
Merge pull request #214 from mikitex70/master

Add plantuml_rst plugin
Added Image metadata attribute for article to specify a custom representative image.
Add unit test for custom image
Update readme to clarify behaviour of plugin and specifying a custom image