Mention missing features in beta1 release

[ricardoV94]

We should look at the milestones for beta2 and V4, for a comprehensive list of missing features and write them in the migration guide or release notes.

From the top of my mind the following are not working yet:

• Cholesky* dists
• Mixtures
• Timeseries
• find_MAP
• Variational inference
• GPs
• EllipticalSlice
• BaseStochasticGradient
• sample_predictive_w
• Probably more...

Otherwise we will have a lot of people opening unnecessary issues

[michaelosthege]

And ideally we should link these tickets right in the exception message.