Add CONTRIBUTING.md explaining how to run the tests

[duvet86]

Hello and thanks for the library.

My team and I would like to submit a PR to enable dynamic configuration, similar to what auth0 does with their angular-sdk.

Can you please include a CONTRIBUTING.md explaining how to run the tests. Because apparently the tests fail if I am not replacing the TEST_GTM_ID with a valid key. Is that expected?

Thank you,
Luca

[duvet86]

bump, or we will have to fork it and create a different package.

[mzuccaroli]

Sorry for being late , I'm the only mantainer and I'm busy.
While i work on a official CONTRIBUTING.md, I think this is not an expected behavior.

Generally, as you can se, the unit tests are quite small and not updated. I found that the only reliable test are tricky and manual: you need to build the lib, copy the build artifacts in the "node_modules/angular-google-tag-manager/" folder of the demo application and tun it with a real GTM ID.

[duvet86]

Grazie Marco,
sorry for pushing you. I'll submit a PR with the dynamic config very soon.

Thanks for your work,
Luca