Add a PR template, update README formatting

[jsiirola]

Fixes #N/A .

Summary/Motivation:

This PR adds a Pull Request Template (derived from Pyomo's) and cleans up the markdown formatting in the README.

Changes proposed in this PR:

• Add (this) PR template
• Clean up markdown formatting so the README renders correctly on GitHub

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1. I agree my contributions are submitted under the BSD license.
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