Standardize pandas distribution lists
As the project and the community grows, I think it'd be good to keep some consistency among the different channels to communicate. Currently we have:
- pydata@googlegroups.com (broader than pandas, but it's kind of our users list)
- pandas-dev@python.org
- pandas-core-team@googlegroups.com
- pydata/pandas on Gitter for IM (I guess it's more intended for devs, but a mix of dev/users in practice)

I don't have a preference on the platform (google groups, mailman, discourse if it's free for open source projects,...), but I think it'd make things simpler and better if we had:
- pandas-users@...
- pandas-contributors@...
- pandas-maintainers@...

And same channels in Gitter. The pandas-maintainers ones should be private for the active maintainers (both the list and the gitter channel).

I think the names would make it clear what's expected to be discussed, and should help keep each of them focused (I'd avoid pandas-dev, since I think for beginners it can be ambiguous if a pandas dev is a person working with pandas or on pandas).

CC: @pandas-dev/pandas-core 