Fix link to CHANGELOG

[koppor]

keep a changelog really recommends to have a CHANGELOG.md file in the root of the repository. This is very common and helps open source people finding the information in new repositories easily.

I agree that it is important to have it nicely rendered at http://ctongfei.github.io/progressbar/changelog/.

In commit fdf2baa, the changelog was moved to docs/ and renamed to changelog.md. This PR fixes the link.

I would recommend to revert the move. And then either

1. maintain the file twice (CHANGELOG.md and docs/changelog.md)
2. Create a script copying the CHANGELOG.md to docs/changelog.md during some gh-pages deployment step. This is possible by using CircleCI and some magic shell script. See https://github.com/latextemplates/LNCS/blob/d44efccf2a3b6623733311b5b2860589bc175cc4/.circleci/config.yml#L26 for a quick example.

[ctongfei]

Thanks. I moved the changelog from docs/changelog.md to the original CHANGELOG.md at the home directory, and created a symlink in docs/. This does not seem to influence mkdocs build -- the documentation site could still be rendered correctly.