Update UDF contributing docs

[danieldeleo]

Update docs to address the issue that was surfaced in thie PR

[sdepablos]

Related to this, on the UDF documentation there's an explanation on how to add the UDFs to your private project, but the explanation should be improved for the cases the UDF function uses a JS function, as creating the SQL UDF in your project is not enough, you need to upload the JS file to a bucket and change the JS route.

[danieldeleo]

Thanks! good catch

[sdepablos]

I've created a PR #218 to address it. Let me know if there's something wrong or I can improve it somehow.

[danieldeleo]

Thank you for starting the work on this! I need to evaluate the best method of having users locally test with js libs. It won't be as easy as having them copy js files to their own GCS bucket because we use webpack to bundle everything into one minified js file. I'm currently testing a method that allows users to run the Cloud Build test pipeline in their own GCP project. Will also evaluate building locally using the cloud-build-local tool but that requires having Docker installed on the local environment.

[sdepablos]

Yes, I could only the part about adding the UDF to your private project, not the testing part.