Add changelog to OEE branch

[rmoch]

Expected Behavior

We should log our changes to openedx-docker images but also
to edX original edx-platform behavior (xblocks, fonzie)

Solution

Add a CHANGELOG.md file and complete with all significant changes we made
Add related CI step

see: https://github.com/openfun/fonzie/blob/master/CHANGELOG.md

[jmaupetit]

Fixed in #112