doc: install instruction: explicit about removing scikits.statsmodels

[josef-pkt]

see also #405 and #661

this should be very explicit in the release notes and install instructions, to reduce the chance that users get the wrong package.

[jseabold]

It's in the release notes and it's not really an install instruction. The namespace is gone. I'm not sure there are enough people doing this as time goes on to warrant much more noise about it.

[josef-pkt]

We haven't heard of any problems in a while, closing