Add CHANGELOG.

CHANGELOG.md

@@ -140,6 +140,9 @@ Changes in Coq:
       authoritative injection.
   - `auth_both_valid` → `auth_both_valid_2`
   - `auth_valid_discrete_2` → `auth_both_valid`
+* Add the camera `ufrac` for unbounded fractions (i.e. without fractions that
+  can be `> 1`) and the camera `ufrac_auth` for a variant of the authoritative
+  fractional camera (`frac_auth`) with unbounded fractions.
 
 ## Iris 3.1.0 (released 2017-12-19)