add references for german and english inventories

metrics/metrics.py

@@ -98,6 +98,7 @@ voiceless_consonants: Set[Category] = {"T", "S"}
 consonants: Set[Category] = {"J", "R", "L", "N", "S", "Z", "D", "T"}
 obstruents: Set[Category] = {"S", "Z", "D", "T"}
 
+# ref doi:10.1017/S0025100304001768
 english_sounds: Set[Sound] = {
     "p",
     "b",
@@ -136,6 +137,8 @@ english_sounds: Set[Sound] = {
     "Q",
     "A",
 }
+
+# ref doi:10.1017/S0025100300004084
 german_sounds: Set[Sound] = {
     "2",
     "2_o",