updated changelog
Add scheduling feature in CHANGES.txt

- Add scheduling feature to `CHANGES.txt`
- Minor doc fix for schedule rule.
Merge pull request #624 from punchagan/scheduling-changes.txt

Add scheduling feature in CHANGES.txt
fix tests
Merge branch 'master' of github.com:ralsina/nikola
added test for math
fix tests for py3
added demo page for longform text
added sample chart page
Make simple styles work in charts (Issue #628)