Annotate issues with closed reasons.

[zanerock]

For closed issues, we want to mark why closed. In general, we will support annotating issues within the issue body / description. First off is 'closed reason' written as closed reason: [done|duplicate #XX|error|rejected]. Text on the next line, which is not itself an annotation, will be treated as further explicative, esp. recommended for 'rejected' issues.

We want to document this, and also implement addition of annotation for in issue submission.