DOC: adopt label convention for issue annotation
this is the schema from astropy

https://github.com/astropy/astropy/wiki/Issue-labeling-convention

Numpy/scipy are considering this

might make issue navigation easier

making this issue for comments on these labels (I am for accepting them almost as is)

---

`package-novice` - accessible to someone who has no knowledge of astropy internals and can be done without this knowledge.
`package-intermediate` - accessible to someone who has at least good experience in using the package and understands the issue well.
`package-expert` - accessible only to people intimately familiar with the sub-package/functionality.
`effort-low` - a few hours at most
`effort-medium` - a few days at most
`effort-high` - longer term
The effort should be indicated for the package-... level specified (so the effort may be lower for a more experienced developer). We also use the following tags:

`domain-specialist` - indicates that knowledge of the specific domain (e.g. cosmology, coordinate transformation) is needed.
the we should doc these on the contributors page
