static badge on about page #107

This provides instructions to add a simple static badge (created by @ohoang) to a README.md file.

* As a static badge, it makes no difference if GitHub caches it, so HTTPS isn't required.
* A handler for a dynamic badge could be at the same location, but the URL would change anyway, being HTTPS.
* Ideally the badge would be available from package pages, which would allow it to provide the exact Markdown code, instead of instructions to replace the <import path>. This is common practice for sites providing badges, and would promote use of the badge (but we may not want to promote it heavily until we have a dynamic badge in place).