Improve logical geometry formatting

Logical geometry is complicated, so the string formatted output is
verbose. This means that flow tree dumps often go well beyond the
edge of the terminal screen. With a simple notation, we can shorten the
output and make it slightly easier to read. This notation also makes it
more similar to the formatted output of Rect, Point2D, and Size2D.
readme: Embiggen the name of the project
Merge pull request #3767 from pcwalton/readme-header

readme: Embiggen the name of the project
auto merge of #3748 : mrobinson/servo/trimmer-flow-tree-output, r=mbrubeck

Logical geometry is complicated, so the string formatted output is
verbose. This means that flow tree dumps often go well beyond the
edge of the terminal screen. With a simple notation, we can shorten the
output and make it slightly easier to read. This notation also makes it
more similar to the formatted output of Rect, Point2D, and Size2D.