CHANGELOG.md: create changelog file of sheepdog

sheepdog is a project under heavy development, so sheep and dog breaks
compatibility occasionally. This makes trouble because external
software, from administrators' home made scripts to OpenStack Cinder [1],
makes assumptions about their interface.

This patch adds a new file CHANGELOG.md for describing important
changes introduced during development. Users can read this file and be
notified of the changes.

I want developers change this file when they make interface
changes. Especially:
1. change of command line
2. change of output to stdout and stderr, especially raw output
3. change of return values of commands
should be noted in the file.

[1]: https://github.com/openstack/cinder/blob/master/cinder/volume/drivers/sheepdog.py

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>