#21: README.md, define 'system partition'
#21: README.md, link text
#21: README.md, add link
#21: README.md, describe raspberry pi system partition
#21: README.md, include detail to setup 'system partition'
#21: README.md, describe 'cmdline.txt'
#21: README.md, minor language changes
#21: README.md, further describe boot sequence
#21: README.md, minor language changes
#21: README.md, minor language changes
#21: README.md, minor language changes
#21: README.md, minor language changes
Merge pull request #30 from jeff1evesque/doc_usb_system_partition

#21: Add 'USB System Partition' Subsection