add a CHANGELOG

CHANGELOG.md

+In this changelog, we document "large-ish" changes to Iris that affect even the way the logic is used on paper.
+We do *not* document changesd of functions names or similar Coq-only API changes.
+Changes marked [*] still need to be ported to the Iris Documentation.
+
+## Iris 2.0
+
+[*] Make the core of an RA or CMRA a partial function.
+
+## Iris 2.0-rc1
+
+This is the Coq development and Iris Documentation as submitted to ICFP.