Update the link to docs

See https://github.com/shoreditch-ops/artillery/issues/49

Update email link

Fix link to website

Add build status badge

Extract some content into wiki pages

Add link to Gitter chatroom

Add link to plugin guide

- Include description of -k/--insecure
- Include description + screenshot for `minigun report`
- I like hearing from users :) 

- Now with more dinosaurs

- Fix link to script example.

Tweaks to "performance" section.

- Document basic HTTP features
- Document WebSocket support

Document setting custom HTTP headers

Link to CONTRIBUTING.md

phases are now specified with "duration" and "arrivalRate" fields, e.g.:

{"duration": 20, "arrivalRate": 10}