Create documentation

[thohan88]

Data

• Describe data sources (origin, frequency of updates, attribution, etc)
• Describe cleaning procedure of data sources where relevant (e.g. MSIS)
• Add a "expected data standard" section (formats, lookup keys, etc)

Contributing

• Add a section for others to contribute their own data
• Add a section for labeling missing data (e.g. MSIS for data pre 25th of March)