Update CHANGELOG.md

CHANGELOG.md

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 Changelog
 =========
 
+## Version 3.1
+*(2018-11-13)*
+- Update naming of methods and view attributes. While this unfortunately is another breaking change, the updated naming should be more future-prove.
+- Add a customizable bottom line below the header (off by default).
+- Add a customizable vertical line between the time column and the events area (off by default).
+- Add option `horizontalScrollingEnabled` to completely disable horizontal scrolling. This is helpful for static calendar views that always show a particular time span. 
+- Save number of visible days on orientation change, so that it doesn’t go back to the initial value after a device rotation.
+
 ## Version 3.0
 *(2018-11-04)*
 - Refactor the project for improved understandability (contributions welcome!)