Add demo directory with jupyter notebook implementation examples

[chrissimpkins]

Approach:

• Pull repo
• install dependencies with make install in root project directory
• jupyter notebook in root project directory
• navigate to demos directory
• select desired demo jupyter notebook that addresses X and execute it for a demonstration of how X is implemented

[chrissimpkins]

https://github.com/googlefonts/fontelemetry/commit/75f4b32fff75012634b765e0fa46c7d319d6bf42 started