Add some info to README

[HerrSubset]

Changes to main README:

• Link to slack now points directly at the correct channel
• Added link to production site
• Added (extra) link to Travis
• Added small setup section that links to the front-/backend README

Changes to backend README

• Write a little more about the gradle wrapper
• Some formatting

[HerrSubset]

You can check what the readme looks like on my fork.