Add leap year note

Author: ErikSchierboom

exercises/baffling-birthdays/instructions.md

@@ -10,6 +10,9 @@ To do this, you need to:
 - Calculate the probability of at least one match for different group sizes.
 
 ~~~~exercism/caution
-The birthday paradox assumes that birthdays are uniformly distributed within a single year.
-While the year should vary between different random birthdate generation calls, all birthdates within a single call must share the same year.
+The birthday paradox assumes that:
+- Birthdays are uniformly distributed within a single year
+- That year has 365 days (no leap years)
+
+While the year should vary between different random birthdate generation calls, all birthdates within a single call _must_ share the same year.
 ~~~~