Update README to use pydata instead of pystatsmodels google group

I'm pretty sure PyData is the main mailing list for pandas now, e.g., as seen on the website:
http://pandas.pydata.org/community.html
Merge pull request #8047 from shoyer/patch-1

Update README to use pydata instead of pystatsmodels google group
PERF: StataReader is slow due to excessive lookups for missing vales

Previous versions of StataReader did not correctly check for missing values.
This was fixed in a previous PR, but these checks had previously been
implemented on a value-by-value basis.  This has now been changed to a
vectorized version that is orders of magnitude faster.

Additionally, a benchmark was added to monitor performance issues in the
future.

Closes #8040