guide on installing requirements, running flake8

Author: Conengmo

.github/CONTRIBUTING.md

@@ -44,31 +44,37 @@ Whenever possible, please also include a [short, self-contained code example](ht
 
 First of all, thanks for your interest in contributing!
 
-- If you are new to git/Github, please take check a few tutorials
-  on [git](https://git-scm.com/docs/gittutorial) and [GitHub](https://guides.github.com/).
-- The basic workflow for contributing is:
-  1. [Fork](https://help.github.com/articles/fork-a-repo/) the repository
-  2. [Clone](https://help.github.com/articles/cloning-a-repository/) the repository to create a local copy on your computer:
-    ```
-    git clone git@github.com:${user}/folium.git
-    cd folium
-    ```
-  3. Create a branch for your changes
-    ```
-    git checkout -b name-of-your-branch
-    ```
-  4. Make change to your local copy of the folium repository
-  5. Make sure the tests pass:
-    * in the repository folder do `pip install -e .`  (needed for notebook tests)
-    * run `python -m pytest tests`
-    * resolve all errors
-  6. Commit those changes
-    ```
-    git add file1 file2 file3
-    git commit -m 'a descriptive commit message'
-    ```
-  7. Push your updated branch to your fork
-    ```
-    git push origin name-of-your-branch
-    ```
-  8. [Open a pull request](https://help.github.com/articles/creating-a-pull-request/) to the python-visualization/folium
+If you are new to git/Github, please take check a few tutorials
+on [git](https://git-scm.com/docs/gittutorial) and [GitHub](https://guides.github.com/).
+
+The basic workflow for contributing is:
+
+1. [Fork](https://help.github.com/articles/fork-a-repo/) the repository
+2. [Clone](https://help.github.com/articles/cloning-a-repository/) the repository to create a local copy on your computer:
+   ```
+   git clone git@github.com:${user}/folium.git
+   cd folium
+   ```
+3. Create a branch for your changes
+   ```
+   git checkout -b name-of-your-branch
+   ```
+4. Install the dependencies listed in `requirements.txt` and `requirements-dev.txt`.
+5. Install Firefox, download [geckodriver](https://github.com/mozilla/geckodriver/releases) 
+   and put it in the PATH.
+6. Make changes to your local copy of the folium repository
+7. Make sure the tests pass:
+   * in the repository folder do `pip install -e .`  (needed for notebook tests)
+   * run `python -m pytest tests`
+   * run `flake8 folium --max-line-length=120`
+   * resolve all errors
+8. Commit those changes
+   ```
+   git add file1 file2 file3
+   git commit -m 'a descriptive commit message'
+   ```
+9. Push your updated branch to your fork
+   ```
+   git push origin name-of-your-branch
+   ```
+10. [Open a pull request](https://help.github.com/articles/creating-a-pull-request/) to the python-visualization/folium