Add info about enviroment

CONTRIBUTING.md

@@ -11,15 +11,18 @@ Also, please check the [Gensim FAQ](https://github.com/RaRe-Technologies/gensim/
 1. <a href="https://github.com/RaRe-Technologies/gensim/fork">Fork the Gensim repository</a>
 2. Clone your fork: `git clone https://github.com/<USERNAME>/gensim.git`
 3. Create a new branch based on `develop`: `git checkout -b my-feature develop`
-4. Implement your changes
-5. Check that everything's OK in your branch:
+4. Setup your enviroment
+   - Create virtualenv: `pip install virtualenv; virtualenv new_env; source new_env/bin/activate` 
+   - Install gensim test dependencies in editable mode: `pip install -e .[test]`
+5. Implement your changes
+6. Check that everything's OK in your branch:
    - Check it for PEP8: `tox -e flake8`
    - Build its documentation (works only for MacOS/Linux): `tox -e docs`
    - Run unit tests: `tox -e py{version}-{os}`, for example `tox -e py27-linux` or `tox -e py36-win` where
       - `{version}` is one of `27`, `35`, `36` and
       - `{os}` is either `win` or `linux`
-6. Add files, commit and push: `git add ... ; git commit -m "my commit message"; git push origin my-feature`
-7. [Create a PR](https://help.github.com/articles/creating-a-pull-request/) on Github. Write a **clear description** for your PR, including all the context and relevant information, such as:
+7. Add files, commit and push: `git add ... ; git commit -m "my commit message"; git push origin my-feature`
+8. [Create a PR](https://help.github.com/articles/creating-a-pull-request/) on Github. Write a **clear description** for your PR, including all the context and relevant information, such as:
    - The number of the issue that you fixed, e.g. `Fixes #123`
    - Motivation (why did you create this PR, what functionality does it seek to improve, what was the problem + an overview of how you fixed it, whom does it help and how should people use it…)
    - Any other useful information (links to other related Github or mailing list issues and discussions, benchmark graphs, academic papers…)