Add badge for Github Actions CI

[Empty2k12]

In https://github.com/rust-lang/crates.io/blob/master/src/tests/badge.rs, there should be an action to set a (/ multiple?) Github Actions CI badge(s).

The format of the badge URL can be found here: https://help.github.com/en/github/automating-your-workflow-with-github-actions/configuring-a-workflow#adding-a-workflow-status-badge-to-your-repository

Necessary parameters are OWNER, REPOSITORY, WORKFLOW_NAME or WORKFLOW_FILE_PATH. Optional parameters are branch and event.

This is an example of a Rust Workflow whose badge a user might want to add to crates.io: https://github.com/Empty2k12/rust-github-actions-test/workflows/Rust/badge.svg?branch=master, which looks like

[carols10cents]

#1838 is in progress to implement this.

[Empty2k12]

My bad.

Since there is no issue open for #1838 I vouch to keep this open. Reading through #1838, other people have opened issues with the exact same issue since they didn't find an issue.

[Turbo87]

we've today decided to go in a different direction which should probably resolve this problem. more details are available in #2436