Add comment on android.nonFinalResIds=false

gradle.properties

@@ -40,4 +40,12 @@ android.useAndroidX=true
 kapt.incremental.apt=true
 
 android.nonTransitiveRClass=true
+
+# Using non-final resource IDs is supposed to improve compilation performance.
+# The idea is that they must be unique among libraries,
+# so in case of a collision recompilation is reqiured.
+# However, in practice, after switching this to true (now default) I see
+#   * no difference in incremental build times, and
+#   * slight increase in a full build time,
+# so let's leave it as it is for now.
 android.nonFinalResIds=false