Add explanation for use of sf package

Add link to the Wikipedia entry for Simple Features, and statement
about other software that implement the Simple Features
Access standard, to provide context for students on why
the `sf` package is used for vector data in these lessons

Author: mneilson-usgs

episodes/06-vector-open-shapefile-in-r.Rmd

@@ -52,10 +52,13 @@ into a single plot.
 
 ## Import Vector Data
 
-We will use the `sf` package to work with vector data in R. `sf` stands for simple features.
-We will also use the `terra` package, which has been loaded in previous episodes, so we can
-explore raster and vector spatial metadata using similar commands. Make sure
-you have the `sf` library loaded.
+We will use the `sf` package to work with vector data in R. `sf` stands for
+[simple features](https://en.wikipedia.org/wiki/Simple_Features), an international standard for
+representing spatial data that is used widely by databases (e.g., PostGIS) and other open source
+geospatial software (e.g., GDAL). We will also use the `terra` package, which has been loaded
+in previous episodes, so we can explore raster and vector spatial metadata using similar commands.
+
+Make sure you have the `sf` library loaded.
 
 ```{r load-sf, results="hide", eval=FALSE, message=FALSE}
 library(sf)