Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -6,3 +6,7 @@
   * Due to the way YAML treats Norway (yes, search for that, it's funny), the `NO` 2-letter-key was quoted. See [issue 48](https://github.com/rugk/crops-parser/issues/48).
   * Palestine was manually assigned the `PS` code [as per this source](https://github.com/lukes/ISO-3166-Countries-with-Regional-Codes/blob/master/all/all.csv), so it could be mapped without an alpha-3 code.
   * That you get an error for "WARNING: No language code for China could be found. Skip." is to be expected, because the FAQ language mapping defines no 2-letter language for this entry, but for `China, mainland` instead. To not wrongly aggregate already aggregated data again, this should stay like it is. (I.e. we should not define the `CN` for "China" again, because it has been assigned to `China, mainland` according to the FAO data.)
+
+## Suggesting new OSM crops
+
+Please adhere [to our criteria for including OSM crops](https://github.com/rugk/crops-parser/wiki/Inclusion-conditions).