Improved Profile illustration

[alainchiasson]

While trying to create a thread profile, I had a hard with the visualization in DesignOfThreadLib.md. I'm a Sysadmin/IT guy but do write tech docs - 3d printing is a hobby, and "thread specifications" has not really been dinner conversation ( to be clear, it still isn't )

I'll add a pull request of the docs I added, I'm looking for early feedback, as I still have changes to make ( eg: crop the png images, confirm a few assumptions ). I also made assumption on certain things - but have not verified in the code ( "-int", "-ext" always appended ? )