Explain how to connect to angostura BigQuery tables

[asciidiego]

According to #52, it seems that there is an angostura_connection.json file that can be used to, well, connect to Angostura (BigQuery tables).

There is no place that explains how this works. Since I have doubts about it, I think others will too. This issue is supposed to be "fixed" by a PR that explains how that works.

Perhaps @marianelamin can help us with this? :)

[marianelamin]

If this file or content does not exist, one of the tests (pulls data in from Angostura) will fail.
Class AngosturaLoader, located in https://github.com/code-for-venezuela/c4v-py/blob/master/src/c4v/data/angostura_loader.py needs access to this file.
More to come on this.