Release timeline for 0.6.0

[alanmcruickshank]

Following on from #542 - this tracks any discussion for larger and breaking changes to be introduced in the next version. I've not included a github project board at this stage because we didn't use it very much for 0.5.0.

As of writing, most of the planned changes appear to be incremental, however the next big items could be:

• Severity levels: Add severity level for lint violations #201
• More advanced template ignoring Templated jinja is not ignored #831
• Missing element fixes More helpful error message when missing statement seperator (;) #830
• Threading --threads argument for sqlfluff lint #642

My assumption is that we'll have a series of minor releases before we get to this point however.