Update README

[twollnik]

The README still needs quite a bit of work. TODOs:

• [x] Better description of features (include screenshots)
• [x] Add section about workflows (starting a new project, checking instrument performance, using existing data)
• [x] Add text to section "How is the data stored?"
• [x] set lifecyle badge to "maturing"
• [ ] add section "How to contribute"
• [ ] improve setup instructions
• [ ] maybe add deployment instructions

[thomas-muench]

Update on the setup instructions:

On my Mac machine (MacbookPro 2013 with High Sierra) I could successfully install and run the app locally without the need for installing any of the build-essential libxml2-dev libssl-dev libcurl4-openssl-dev dependencies (or rather Mac substitutes thereof).

The only difference to the current description is that I had to run the app with Rscript -e "shiny::runApp('R', launch.browser = TRUE)" to have the app actually open in my browser.

I would thus suggest to have setup instructions separate for Linux and Mac.