chore(changelog): add breaking changes migration guide

CHANGELOG.md

@@ -21,9 +21,9 @@
 
 ### BREAKING CHANGES
 
-* **nb-icon:** `NbIconLibraries.getSvgIcon` and `NbIconLibraries.getIcon` could return null if icon wasn't found and won't throw.
-* **Angular:** Nebular packages now require Angular 10+. Nebular packages no longer include ESM5 and FESM5 bundles as per Angular  Package Format 10.
-* **NbBasePicker**: Angular lifecycle methods moved to `NbBasePickerComponent`.
+* **nb-icon:** `NbIconLibraries.getSvgIcon` and `NbIconLibraries.getIcon` could return null if icon wasn't found and won't throw. As methods don't throw now, you need to possible `null` return value.
+* **Angular:** Nebular packages now require Angular 10+. Nebular packages no longer include ESM5 and FESM5 bundles as per Angular Package Format 10.
+* **NbBasePicker**: Angular lifecycle methods moved to `NbBasePickerComponent`. If you use `NbBasePicker` as a base class of your custom picker change it to `NbBasePickerComponent`.
 
 <a name="5.1.0"></a>
 # [5.1.0](https://github.com/akveo/nebular/compare/v5.0.0...v5.1.0) (2020-06-03)