improve contribution documentation

[schuderer]

Missing info in CONTRIBUTING:

• how to activate the environment ($ pipenv shell, see README)
• setting the upstream and refreshing/pulling from it: from https://scikit-learn.org/dev/developers/contributing.html
• using google style, particularly docstrings: https://google.github.io/styleguide/pyguide.html#381-docstrings

Add the upstream remote. This saves a reference to the main scikit-learn
repository, which you can use to keep your repository synchronized
with the latest changes:

$ git remote add upstream https://github.com/scikit-learn/scikit-learn.git

...

It is often helpful to keep your local branch synchronized with the latest
changes of the main scikit-learn repository:

$ git fetch upstream
$ git merge upstream/master

NOTE: it's preferrable to (if possible) rebase instead of merging (merging creates a lot of meaningless merge commits). In our doc, the last suggested command should probably be:

$ git rebase upstream/master

[gosiarorat]

i am taking the issue