Add status of regions

Author: tmandry

src/traits/regions.md

@@ -1,3 +1,9 @@
 # Region constraints
 
-*to be written*
+*To be written.*
+
+Chalk does not have the concept of region constraints, and as of this
+writing, work on rustc was not far enough to worry about them.
+
+In the meantime, you can read about region constraints in the
+[type inference](../type-inference.html#region-constraints) section.