MAINT: update changelog for 3.2 release
## Version 3.2.0 (unreleased)
### Enhancements
+
+- Selector values can be used directly in expressions (#1599)
+- Top-level chart repr is now truncated to improve readability of error
+ messages (#1572)
+
### Bug Fixes
+
+- top-level ``add_selection`` methods now delegate to sub-charts. Previously
+ they produced invalid charts (#1607)
+- Unsupported ``mark_*()`` methods removed from LayerChart (#1607)
+- New encoding channels are properly parsed (#1597)
+- Data context is propagated when encodings are specified as lists (#1587)
+
### Backward-Incompatible Changes
+- ``alt.LayerChart`` no longer has ``mark_*()`` methods, because they never
+ produced valid chart specifications) (#1607)
## Version 3.1.0 (Released June 6, 2019)
