Add CONTRIBUTING.md doc

Author: yuvipanda

CONTRIBUTING.md

@@ -0,0 +1,54 @@
+# Contributing
+
+Contributions to nbresuse are highly welcome!
+
+## Development set up
+
+We recommend using [pipenv](https://docs.pipenv.org/) to make development easier.
+
+1. Clone the git repository:
+
+   ```bash
+   git clone https://github.com/yuvipanda/nbresuse
+   ```
+
+2. Create an environment that will hold our dependencies.
+   
+   ```bash
+   cd nbresuse
+   pipenv --python 3.6
+   ```
+
+3. Activate the virtual environment that pipenv created for us
+
+   ```bash
+   pipenv shell
+   ```
+
+4. Do a dev install of nbresuse and its dependencies
+
+   ```bash
+   pip install --editable .
+   ```
+
+5. Install and enable the nbextension for use with Jupyter Classic Notebook.
+
+   ```bash
+   jupyter nbextension install --py nbresuse --symlink --sys-prefix
+   jupyter serverextension enable --py nbresuse  --sys-prefix
+   jupyter nbextension enable --py nbresuse --sys-prefix
+   ```
+
+6. Start a Jupyter Notebook instance, open a new notebook and check out the memory usage
+   in the top right!
+
+   ```bash
+   jupyter notebook
+   ```
+
+7. If you want to test the memory limit display functionality, you can do so by setting
+   the `MEM_LIMIT` environment variable (in bytes) when starting `jupyter notebook`.
+
+   ```bash
+   MEM_LIMIT=$(expr 128 \* 1024 \* 1024) jupyter notebook
+   ```