Update README.md
Merge pull request #278 from Coeuvre/master

Added `triangle_face` function to `vecmath`
Implemented `AddRoundBorder` for `RoundBorderLineContext`
Implemented `AddRoundBorder` for `RoundBorderLineColorContext`
Implemented `AddColor` for `RoundBorderLineContext`
Arranged type aliases alphabetically
Added `Stroke` trait
Started on `with_round_border_line_tri_list_xy_f32_rgba_f32`
Replaced `RoundBorder` with single number for radius
Added `round_border_width` to `AddRoundBorder`
Fixed doc comment on `round_border_radius`
Working on `with_round_border_line_tri_list_xy_f32_rgba_f32`
Implemented `Stroke` for `RoundBorderLineColorContext`
Added `AddLine` trait
Implemented `AddLine` for `Context`
Fixed bug in `AddLine` impl for `Context`
Added `with_round_border_line_tri_list_xy_f32_rgba_f32` to `triangulation`
Update README.md
Replaced `[f32, ..4]` with the `Color` type alias in `triangulation`

Also changed it to borrowed pointer as argument for consistency.
Put 'use' statements for PI constants in `triangulation`