doc: user guide.
doc: user guide.

- "sdc docker" gets too close to branding.
- edits to first section.

lloydde/sdc-docker,richardkiene/sdc-docker,13W/sdc-docker,lloydde/sdc-docker,owlin/sdc-docker,misterdjules/sdc-docker,joyent/sdc-docker,misterdjules/sdc-docker,bixu/sdc-docker,bixu/sdc-docker,13W/sdc-docker,mehulsbhatt/sdc-docker,richardkiene/sdc-docker,owlin/sdc-docker,mehulsbhatt/sdc-docker,joyent/sdc-docker