add advice about interactive maps

CONTRIBUTION_GUIDE.md

@@ -20,14 +20,22 @@ Pull requests welcome from anyone. Internal contributors are encouraged to push
 
 ## hazards
 
-1. 2021-01 join-admin previously uploading to shinyapps gave this error
+1. exercise chunks plotting interactive maps (with tmap or mapview) need to have unique ids set like this :
+```{r tmap-points1-setup}
+# to avoid issues with interactive maps https://github.com/rstudio/learnr/issues/529
+# IdSeed must be set to be unique in the Rmd, even for mode('plot') to allow user to change
+tmap_mode("plot")
+htmlwidgets::setWidgetIdSeed(1)
+```
+
+2. 2021-01 join-admin previously uploading to shinyapps gave this error
 Error in value[[3L]](cond) : there is no package called 'sf'
 turned out to be an issue with the fill-the-blanks sections lower down
 updating to dev version of packrat locally fixed deployment 
 remotes::install_github('rstudio/packrat')
 see https://github.com/afrimapr/afrilearnr/issues/4#issue-791404707
 
-2. 2020 intro-to-spatial error on deploying to shinyapps
+3. 2020 intro-to-spatial error on deploying to shinyapps
 Error in value[3L] :OGRCreateCoordinateTransformation() returned NULL: PROJ available?
 https://stackoverflow.com/questions/61286108/error-in-cpl-transformx-crs-aoi-pipeline-reverse-ogrcreatecoordinatetrans
 If you save an sf-dataframe with a newer version of GDAL, and then try on a system with an older version of GDAL, the projection info cannot be read properly.