Example metric proposal for describing, calculating, and benchmarking

[j12y]

This came up as an example in building a getting started so submitting here for review:
DevRel-Foundation/wg-resource-aggregation#139

I chose this metric not to be an example of a perfect metric but rather something easy to understand.

• the level of detail in describing how to measure and overcome obstacles
• the benchmarks so that we can compare the metric across programs
• sample scripts and instructions to help somebody be successful in collecting and interpreting the metric

[j12y]

Can see the end result in the blog-post-publish-rate branch: https://github.com/DevRel-Foundation/metrics-index/tree/dev/j12y/blog-post-publish/metrics/activity/blog-post-publish-rate

fyi: @divya-mohan0209 @arun-gupta