Build working group charter document

[Jessie Zhu]

Expected Behavior

We should have a wg charter document to describe the overall vision, mission and scope for each wg, and https://github.com/knative/docs/blob/master/community/WORKING-GROUPS.md should point readers to such doc.

I'm opening an issue for each wg to track this need.

Actual Behavior

There is no high level intro for each wg on WORKING-GROUPS page.

[Jessie Zhu]

/kind doc