Metrics Index | Create Scaffolding

[divya-mohan0209]

Propose some scaffolding for the metrics index:

• pick a single metric we can use as an example
• create a github issue template for metrics
• create evaluation criteria for reviewing metrics
• add a contribution guide for metrics to the repository
• build list of categories for metrics

[divya-mohan0209]

PR #144 opened toward creating an issue template for DevRel Foundation metrics.