Update format.md

Add optional properties  for Polyline:
LINESHAPESIZE, can take values:
none(-1 ?) - Smallest
1 - Small
2 - Medium
3 - Large

STARTLINESHAPE, can take values as ENDLINESHAPE:
none(-1 ?) - Smallest - None
1 - Arrow
2 - SolidArrow
3 - Tail
4 - SolidTail
5 - Circle
6 - Square