Add detailed describtion on labelling step

[jeremyforest]

Its easy to not be sure how/what to label. Add some examples in the detailed description of labelling.

Examples: I want to label the mouse interacting with an object. Use the polygon to label the mouse, another polygon to label the object. Use the rectangle to label the interactions.

Also make a note of some pitfalls. eg triple clicking seems to make a point but makes a two-point polygon which will error out in the next steps of exporting to COCO compatible labels.