relnotes: add 6.7.2

relnotes/6.7

@@ -1,3 +1,14 b''
+= Mercurial 6.7.2 =
+
+Exceptional release following a large performance regression when cloning.
+
+We are setting up automated benchmarks to reduce the likelihood of regressions
+of the sort from happening in the future.
+
+ * phases: avoid a potentially costly dictionary interation in some case
+ * phases: update the phase set as we go during retract boundary
+ * bundle2: make the "hgtagsfnodes" part advisory
+
 = Mercurial 6.7.1 =
 
 Exceptional release following a crash found in delta code that can be triggered