Update contributing.md for Windows contributors

Went through the steps on my Windows development machine and ran into errors (below). Discovered that this was likely Windows specific and wanted to simply update this so others can avoid similar issues.

Also updated some of the bash commands so that they highlighted having an activated virtual environment for consistency.

Complete output (1 lines):
    A GDAL API version must be specified. Provide a path to gdal-config using a GDAL_CONFIG environment variable or use a GDAL_VERSION environment variable.
    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

Author: MCBoarder289

Diff for: contributing.md

@@ -128,17 +128,27 @@ conda activate plotly-dev
 [conda-env]: https://docs.conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html#creating-an-environment-with-commands
 [virtualenv]: http://docs.python-guide.org/en/latest/dev/virtualenvs/
 
-### Install requirements
+### Install requirements - (Non-Windows)
+```bash
+(plotly_dev) $ pip install -r packages/python/plotly/requirements.txt
+(plotly_dev) $ pip install -r packages/python/plotly/optional-requirements.txt
+ ```   
+### Install requirements - (Windows + Conda)
+Because Windows requires Visual Studio libraries to compile some of the optional dependencies, follow these steps to
+complete installation and avoid gdal-config errors.
 
-    $ pip install -r packages/python/plotly/requirements.txt
-    $ pip install -r packages/python/plotly/optional-requirements.txt
+```bash
+(plotly_dev) $ pip install -r packages/python/plotly/requirements.txt
+(plotly_dev) $ conda install fiona
+(plotly_dev) $ pip install -r packages/python/plotly/optional-requirements.txt
+```
 
 ### Editable install of plotly packages
-
-    $ pip install -e packages/python/plotly/
-    $ pip install -e packages/python/chart-studio/
-    $ pip install -e packages/python/plotly-geo/
-
+```bash
+(plotly_dev) $ pip install -e packages/python/plotly/
+(plotly_dev) $ pip install -e packages/python/chart-studio/
+(plotly_dev) $ pip install -e packages/python/plotly-geo/
+```
 This will ensure that the installed packages links to your local development
 directory, meaning that all changes you make reflect directly in your
 environment (don't forget to restart the Jupyter kernel though!).
@@ -147,11 +157,11 @@ environment (don't forget to restart the Jupyter kernel though!).
 
 Run the following commands in your virtual environment to use the
 development version of `FigureWidget`,
-
-    $ jupyter nbextension enable --py widgetsnbextension
-    $ jupyter nbextension install --py --symlink --sys-prefix plotlywidget
-    $ jupyter nbextension enable --py --sys-prefix plotlywidget
-
+```bash
+(plotly_dev) $ jupyter nbextension enable --py widgetsnbextension
+(plotly_dev) $ jupyter nbextension install --py --symlink --sys-prefix plotlywidget
+(plotly_dev) $ jupyter nbextension enable --py --sys-prefix plotlywidget
+```
 To make plotly plots show up in JupyterLab, you also need to [install the plotly jupyterlab extensions][plotly-jl].
 
 [plotly-jl]: https://plot.ly/python/getting-started/#jupyterlab-support-python-35