Updated contributing.md

[dgwyer]

Proposed Changes
Updated contributing.md to include how to build readme.md. Instructions have been added including library dependency installation.

Types of changes
What types of changes does your code introduce? Put an x in all the boxes that apply:

• Updated docs.

Checklist
Go over all the following points, and put an x in all the boxes that apply:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I am aware that this is an nbdev project, and I have edited, cleaned, and synced the source notebooks instead of editing .py or .md files directly.

[jph00]

Thanks!