Update custom covariate vignette with GIS example

[jshoughtaling]

I recently put together a description of how to use the custom FeatureEngineering workflow to incorporate GIS data into a PLP model, and I would like to contribute this description to the PLP repository to add to the existing (and very useful) vignette! I'm not sure where or how best to integrate it, but I thought it could be a helpful resource to others trying to do similar things with PLP.

The description of my process flow is here: OHDSI/GIS#350 (comment)

[egillax]

@jshoughtaling thank you.

I'll add this example to the feature engineering docs. Might start a series with articles on different custom feature engineering people are doing.