* Reuse intro from contributing.rst
* Removed content already on rst
Simplified CONTRIBUTING.md by removing content that already
exists in contributing.rst. Added Code of Conduct reference
as seen in a few other CONTRIBUTING.md of popular libraries.
* Update contributing.rst
In shortening `CONTRIBUTING.md`, material deleted from `how to contribute` section are added onto `contributing.rst` if it doens't exist on the latter.
* added full link to .rst file + removed 'Full' in 'Full Docum...'
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#### What does this implement/fix? Explain your changes.
As far as I understand it, the fact that `fit` is idempotent means that repeated calls to `fit` with the same data doesn't change the estimator.
The contributing guide was a bit unclear about this.
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In order to fix#11408, this swaps `joblib` and `_joblib`. It however, allows users to access joblib's `Memory` or `Parallel` functionality without accessing `sklearn.externals._joblib` by importing `Memory`, `Parallel`, etc. into `sklearn.utils`.
jeremiedbb
Hanmin Qin