MNT: update CHANGELOG.md

Author: st3107

CHANGELOG.md

@@ -1,5 +1,15 @@
 # Release notes
 
+## Version 2.2.0 -- 2021-06-08
+
+Notable differences from version 2.1.0.
+
+- Add access to Radiation class & functions to change RadiationType, wavelength in PowderPattern and ScatteringData (and hence DiffractionDataSingleCrystal) classes.
+
+- Fix the custodian_ward when creating a PowderPatternDiffraction: PowderPatternDiffraction must persist while PowderPattern exists, and Crystal must persist while PowderPatternDiffraction exists.
+
+- Add 3D Crystal viewer `pyobjcryst.crystal.Crystal.widget_3d`.
+
 ## Version 2.1.0 -- 2019-03-11
 
 Notable differences from version 2.0.1.