Remove unneeded gitignore
Add instructor notes and tweak README
Rectangles can be created with a length and width
Rectangles can calculate their perimeter
Rectangles can calculate their area
Circles can be created with a radius
Circles can calculate their circumference
Circles can calculate their area
Triangles can be created with three side lengths
Triangles can calculate their perimeter
Triangles can calculate their area
Triangles can determine whether they are valid
Add comment on let vs. before
Bonus validation for rectangles