New readme

[tassaron]

My goal was to make the first impression more appealing to the current "target audience" (end user making videos for podcasts or speeches)

Changes

• Added a screenshot to the top
• Moved technical details (dependencies etc) under installation instructions
• Commandline snippets are more relevant to the classic use-case (first snippet doubles as test command)
• Added a link to the wiki page (I may write some more wiki pages for documentation in the future)
• Removed the mention of avp --test because the feature is not useful yet