update additional keys example in vignette

Author: mgyliu

vignettes/epiprocess.Rmd

@@ -200,17 +200,21 @@ ex3 <- jhu_csse_county_level_subset %>%
 attr(ex3,"metadata") # geo_type is county currently
 ```
 
-Now we add state (MA) as a new column and a key to the metadata. Reminder that lower case state name abbreviations are what we would expect if this were a `geo_value` column.
-```{r}
+Now we add `state` (MA) and `pol` as new columns to the data and as new keys to the metadata. Reminder that lower case state name abbreviations are what we would expect if this were a `geo_value` column. 
 
+```{r}
 ex3 <- ex3 %>% 
   as_tibble() %>% # needed to add the additional metadata
-  mutate(state = rep(tolower("MA"),6)) %>%
-  as_epi_df(additional_metadata = list(other_keys = "state"))
+  mutate(
+    state = rep(tolower("MA"),6),
+    pol = rep(c("blue", "swing", "swing"), each = 2)) %>%
+  as_epi_df(additional_metadata = list(other_keys = c("state", "pol")))
 
 attr(ex3,"metadata")
 ```
 
+Note that the two additional keys we added, `state` and `pol`, are specified as a character vector in the `other_keys` component of the `additional_metadata` list. They must be specified in this manner so that downstream actions on the `epi_df`, like model fitting and prediction, can recognize and use these keys.
+
 Currently `other_keys` metadata in `epi_df` doesn't impact `epi_slide()`, contrary to `other_keys` in `as_epi_archive` which affects how the update data is interpreted.
 
 ## Working with `epi_df` objects downstream