Add custom tests instructions for specific packages.

[Annosha]

I was going through the packages individually for the opentelemetry-js repo. Compiled and ran test. Some packages:
Opentelemetry-context-zone have custom test scripts i.e. "test:browser": "karma start --single-run",.
Do we need to add this information about running custom test script for beginners in README.md?

Likewise, for following repositories:
opentelemetry-context-zone-peer-dep
opentelemetry-sdk-trace-web
@pichlermarc

[pichlermarc]

@Annosha I think this can be helpful. If needed it should only go into the top-level CONTRIBUTING.md though.

It may be good to add the info that this test script exists if the package is also expected to run in the browser. It only runs browser-specific tests and tests that are shared between browser and Node.js