Add docs on registering custom shape files

vignettes/ggautomap.Rmd

@@ -232,3 +232,47 @@
   labs(x = NULL, y = NULL) +
   theme_void()
 ```
+
+## Loading custom shape files
+
+Before proceeding to load custom shape files, check the R package [rnaturalearth](https://github.com/ropensci/rnaturalearth) which contains maps for world maps and countries.
+
+A shape file (`.shp`) is required, for example the Australian specific maps can be retrieved from [ABS Digital boundary files](https://www.abs.gov.au/statistics/standards/australian-statistical-geography-standard-asgs-edition-3/jul2021-jun2026/access-and-downloads/digital-boundary-files).
+
+First read in the shape file using the `sf` package:
+
+```{r, eval=FALSE}
+sf <- st_read(map_files)
+```
+Shape files can be subset in a similar fashion to data.frames using `dplyr`:
+
+```{r, eval=FALSE}
+# subset to only include Victorian postcodes only
+sf_vic <- sf::st_read(map_files) %>%
+    mutate(postcode = as.integer(POA_CODE21)) %>%
+    filter(POA_CODE21 >= 3000 & POA_CODE21 <= 3999)
+```
+
+After reading in the shape file, register with `cartographer` package:
+```{r, eval=FALSE}
+cartographer::register_map(
+    "sf.vic", # the map name
+    data = sf_vic, # this is the object we subsetted above
+    feature_column = "postcode", # data column to include
+)
+```
+
+Check the map has been registered:
+```{r, eval=FALSE}
+> cartographer::feature_types()
+ [1] "maps.italy"                    "rnaturalearth.countries_hires" "sf.vic"                        "maps.lakes"                   
+ [5] "rnaturalearth.countries"       "rnaturalearth.australia"       "maps.nz"                       "maps.world"                   
+ [9] "maps.state"                    "sf.nc"                         "maps.france"      
+```
+
+To use the registered shapefile:
+```{r, eval=FALSE}
+...
+  geom_boundaries(feature_type = "sf.vic") +
+...
+```