Add estimator creation section on developer/contributor guide

[salvisolamartinell]

We should add a section on creating estimators on the developer/contributor guide.

• Not to do any logic on the constructor.
• Make constructor parameters public and do not replace them in the algorithm (maybe it's okay to modify a RandomState by using it, but not to change it for another one, and it's not ok to change a seed for a random_state, as clone()'s will fail then).
• How to add it to the GridSearchCV tests.

[javicid]

Everything in #225 should be also added to the estimator creation guide.