Add instructions to the contributing guide for tags

[droumis]

For now (until something changes with #407), we need to add instructions to the contributing guide for adding a category tag for new projects to the doc/tags.yml file when someone creates a new example PR. This is required for a project to appear on the homepage.

<project name>:
  category: [<category 1>, <category 2>]

For example, if someone was contributing the world_cup example for the first time, they should add:

world_cup:
  category: [Sports]

Allowed tags are currently hardcoded:

• Featured
• Geospatial
• Finance
• Economics
• Mathematics
• Cybersecurity
• Networks
• Neuroscience
• Other Sciences
• Sports