docs: add changelog

CHANGELOG.md

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+# Changelog
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [0.12.0] - 2018-11-15
+### Added
+- The special fragment `#top` is now supported for scrolling to the top, but only if no element with id `top` is found
+- After navigating to an anchor, the respective hash is now appended to the URL using `history.pushState()` to match default browser behavior.
+- When a hashchange event occurs, the polyfill tries to cancel the instant jumping scroll to the new hash, handling it with the smooth scroll instead.
+- When navigating to an anchor, the anchor is now focused.
+- In browsers supporting the optional `preventScroll` argument, the anchor is focused immediately and the focus scroll is prevented by passing this argument.
+- If the browser doesn't support `preventScroll` (e.g. Internet Explorer), the focus is scheduled to happen 450ms after the smooth scroll started so it does not interfere with the smooth scrolling (which caused flickering).
+
+
+### Changed
+- The flag to enforce the polyfill (even if the browser has native support) is now called (`window.__forceSmoothscrollAnchorPolyfill__`). The docs have been updated to reflect this. 
+
+
+### Fixed
+- The polyfill now properly handles Shift/Meta keys and allows for opening links in new windows by shift-clicking instead of preventing it with `event.preventDefault()`
+- The docs website now works in Internet Explorer 9, polyfills for `Element.classList`, `requestAnimationFrame` + an alternative for flexbox layouts have been added