Add nose as requirement in Contributing docs

[asishm]

Currently, the contributing documentation recommends creating a conda environment using conda create -n sklldev -c conda-forge --file conda_requirements.txt, and also recommends running nosetests to run the tests locally, but conda_requirements.txt doesn't list nose as a required library.

I'm not sure if nose should be added in the requirements file though as it's not needed for the core functionality.

[desilinguist]

Good catch! We can either add nose to the requirements or just add modify the conda command to be:

conda create -n sklldev -c conda-forge nose --file conda_requirements.txt which will also install nose along with the rest of the requirements.

[desilinguist]

This was addressed in the v2 release.