Add instructions for virtual environment for conda users

## SUGGESTION
Add instructions for users using ```Anaconda``` python distribution.

## Description
Right now, there are instructions to use ```pip``` to install and create virtual environments for python. There should also be instructions to use ```conda``` to create and use virtual environments. ```conda``` is one of the most popular Package, dependency and environment management software and it comes built in with the ```Anaconda``` python distribution. Hence it would be much better if we have some instructions for it also.

The instructions should be added [here](http://docs.opendatakit.org/contributing/#virtual-environment) along with the ```pip``` instructions.

For reference about what to add please check [here](https://conda.io/docs/user-guide/getting-started.html#managing-environments).

Open the contributing documentation at the virtual-environment section and review the existing pip instructions. Use the linked conda environment guide as reference, then add equivalent Anaconda/conda instructions alongside pip and verify that the documentation clearly explains creating and using an environment.

Written by the indexing model from the issue text.