Expand README to add more context

[ElenaFdR]

At the moment, it's hard for a potential contributor to understand:

• where or how this project is currently being used
• the size of the community
• what's most actively under development (eg, the new Dutch translation is an exciting recent milestone)

without trawling through discussions and open issues in multiple repositories.

This README looks like the best location for a stub product/marketing page for the Standard.

[ElenaFdR]

Status of this issue one year on:

• where the project is being used (done)
• size of the community: not explictly stated, but you can infer it from current users, plus recent discussions and community calls (both easily found from this README)
• exciting news or developments: hard to understand, since the community calls often focus on outstanding issues or maintenance

Proposed MVP solution: add a standing agenda item to community calls to identify/celebrate any recent or ongoing community milestones, with an explicit question of how the community can best contribute to or amplify this.
My impression is that this happens informally, but it's not always documented in this way.

This would have the effect of:

• contributing to the momentum of anything new
• allowing people who couldn't make the call to still see/celebrate progress via the minutes