Linfa's 0.4.0 release (rust-ml#127)

* Bump version 0.4.0, add release notes

* Add t-SNE example to release notes

* Add example for t-SNE and start writing preprocessing example

* Fix typo

* Add example for TF-IDF text preprocessing

* Explain that target construction is omitted and traits changes

* Add contribute section on the new lapack traits

* Link to crates.io in the base crate documentation

* Link to crates.io in the base crate documentation

* Add mnist example

* Run rustfmt

* Fix mnist example

Bump version

Release 0.3.0

Improve code style of decision-tree example

Release 0.2.0