DOC: Add instructions for installing with conda

[drewejohnson]

Closes #494

Make sure you have read over the developer guide to ease
the review process. These include:

• PR fits the project scope
• Code to be committed is DRY and matches the code style of the project
• New and/or changed features are well documented and have adequate testing
• For first-time contributors, you have included your attribution in docs/contributors.rst