[enh] update CHANGELOG.md

CHANGELOG.md

+## v0.52.0 (23/04/2020)
+### Features
+* #807 Add new "Percentage of Average" referential
+* Transfer window now use current referential instead of only relative by UD
+* Add current referential units on the balance display
+### Bugs
+* Fix referentials french translation
+* Fix network "Open in browser" menu
+
 ## v0.51.1 (05/04/2020)
 ### Bugs
 * #802 Fix "Send as source" context menu not appear in Expert Mode on normal received transactions