-c/--cpu-shares flag set as a percentage rather than an absolute value

Want to move to a percentage model for -c/--cpu-shares flag. 0 is a misleading value to represent 100% CPU-shares.

WIth this change,

1) If the user starts without the flag: `sudo docker run -it rhel7 /bin/bash`. The JSON `CpuShares` field should have 100%.
For custom values, user should pass percentage rather than an absolute value e.g. 50%, 100%, 25% etc. The JSON would be updated accordingly.

Just added a comment in the doc to open PR. No code changes as of now.