Write up expectations we have for contributions

We recently got a styleguide and we also have other expectations for contributions like commit message char-length and a green passing testsuite, but it is not written up somewhere. This costs time for the contributor and the reviewer.

I changed our doc a bit which is also linked when you create a PR in a yellow box above (`Please review the guidelines for contributing to this repository.`) and started to mention the things that we expect:
 - mention styleguide
 - mention commit message format

I also changed a point which did not make sense to me:
 - small bugfixes do not need a Jira ticket

Summed up over a year it takes a lot of time to find each extra for a PR created Jira ticket and close it, often people forget to close the ticket after the PR is merged (or do not have the karma for it?). 

My main reason for removing this rule is that we do not act to the rule in the project - the theory differs a lot from the practical day-to-day business here. It also raises the entry barrier for folks which just want to fix a small bug, but have to register an account on Jira first, then verify their email, find out how to create a bug on Jira and then write it and just after that are able to submit a patch for a bug.