Update 2018-09-24-maintenance-release-0.6.2.md
Update 2018-09-24-maintenance-release-0.6.2.md

improved GIF import / export description
added GIF import / export menu image link
added undo known issues.
shortened certain known issues previous descriptions to make them more readable and made the writing style more cohesive.
fixed various typos.
pencil2d/pencil2d.github.io,pencil2d/pencil2d.github.io