Review/test Project 3

[zstumgoren]

• Add project3/README.md
• Review/test (in Codespace)project3/analyzing_data_with_pandas_notebook.ipynb
• Review/test (in Codespace) completed/analyzing_data_with_pandas_notebook_complete.ipynb
• Update teachers_guide.md to include overview of Project 3 and link to relevant notebook and its "completed" file