instructions for python 3.x kernels with conda

this was surprisingly hard to find help for online that was up-to-date and worked with the stacks, contributing here in hopes it'll help others.

docs/using/recipes.md

@@ -98,6 +98,47 @@ USER $NB_USER
 Ref:
 [https://github.com/jupyter/docker-stacks/issues/440](https://github.com/jupyter/docker-stacks/issues/440)
 
+## Add a Python 3.x environment
+
+The default version of Python that ships with conda/ubuntu may not be the version you want.
+To add a conda environment with a different version and make it accessible to Jupyter, the instructions are very similar to Python 2.x but are slightly simpler (no need to switch to `root`):
+
+```
+# Choose your desired base image
+FROM jupyter/minimal-notebook:latest
+
+# name your environment and choose python 3.x version
+ARG conda_env=python36
+ARG py_ver=3.6
+
+# you can add additional libraries you want conda to install by listing them below the first line and ending with "&& \"
+RUN conda env create --quiet --yes -p $CONDA_DIR/envs/$conda_env python=$py_ver ipython ipykernel && \
+    conda clean --all -f -y
+
+# alternatively, you can comment out the lines above and uncomment those below
+# if you'd prefer to use a YAML file present in the docker build context
+
+# COPY environment.yml /home/$NB_USER/tmp/
+# RUN cd /home/$NB_USER/tmp/ && \
+#     conda env create -p $CONDA_DIR/envs/$conda_env -f environment.yml && \
+#     conda clean --all -f -y
+
+
+# create Python 3.x environment and link it to jupyter
+RUN $CONDA_DIR/envs/${conda_env}/bin/python -m ipykernel install --user --name=${conda_env} && \
+    fix-permissions $CONDA_DIR && \
+    fix-permissions /home/$NB_USER
+
+# any additional pip installs can be added by uncommenting the following line
+# RUN $CONDA_DIR/envs/${conda_env}/bin/pip install 
+
+# prepend conda environment to path
+ENV PATH $CONDA_DIR/envs/${conda_env}/bin:$PATH
+
+# if you want this environment to be the default one, uncomment the following line:
+# ENV CONDA_DEFAULT_ENV ${conda_env}
+```
+
 ## Run JupyterLab
 
 JupyterLab is preinstalled as a notebook extension starting in tag