new issue label: `reference`

[josef...@gmail.com]

We, mainly I, use github issues also to keep track of design issues, possible enhancements and references for the background of the statistics or implementation.

`design`, `wishlist` and now also `reference`  are general labels to bring a bit of order into this. 

`type-enh` is a bit similar but already closer to getting an implementation or directly related to improving an existing functionality.

I started `reference` for issues that are mainly adding a reference. We don't have a systematic bibliography yet.

current status

1150 issues

design 221

type-enh 521

wishlist 129

type-refactor 85

type-bug 151

no label 52

https://github.com/statsmodels/statsmodels/labels

(we have closed 981 PRs but only closed 761 issues, 218 of which have label bug, in around 5 years of github history)

Josef