[DOCS] Add documentations

[grgprarup]

• Project Setup docs in README.md Setup web app presentation viewer #8
• Add docs for markdown file format
• Add needed documentation

[koebel]

README.md needs some proof reading... Make sure to spell at least ownCloud properly :)

[koebel]

In my understanding the current version only allows preview of presentation files, not creation or modification. Is that correct? If so, the sentence to create slide presentations directly from markdown files in the description of the extension in README.md can lead to miss-understanding of the features provided. To make it clearer write something like to preview slide presentations that were created in markdown files using the reveal.js syntax

[koebel]

I've added a 1 sentence description plus some keywords in "About" section of this repo. Please have a look.