Documentation for `web_library`

Hey there `rules_closure` team - I'm getting an error trying to setup Bazel with the Closure toolchain:

```
ERROR: /datasync/ui/BUILD:13:1: Checking webfiles in //datasync/ui:popup failed (Exit 1)
ERROR: datasync/ui/popup.html: Referenced datasync.js (/datasync.js) without depending on a web_library() rule providing it
NOTE: Use suppress=["strictDependencies"] to make the errors above warnings
INFO: Elapsed time: 0.371s, Critical Path: 0.02s
INFO: 1 process: 1 worker.
FAILED: Build did NOT complete successfully
```

It would be massively helpful if someone could explain how to use `web_library` in conjunction with `closure_js_library`/`closure_js_binary`