Created CONTRIBUTING documentation and udpated README

@mads379 Please, have a look at it and let me know if there is anything you don't like.

Also, I moved all tickets to Assembla, under the [Scala Search Milestone](https://scala-ide-portfolio.assembla.com/spaces/ae55a-oWSr36hpeJe5avMc/tickets?report[estimate_show]=true&report[id]=0&report[milestone_id_cond]=0&report[milestone_id_val][]=2130873&report[ticket_status_id_cond]=0&report[ticket_status_id_val][]=all_open&report[title]=Active+tickets&report[total_estimate_show]=true&report[total_invested_hours_show]=true&report[working_hours_show]=true). When you close a ticket, you can refer to the ticket by saying ``Fix/Fixes/Fixed #<ticket-number>`` (as described in the in CONTRIBUTING doc).