Update What_Is_a_Font.md

1. Added a new sub section on "Serifs" as this is an early choice to make in font/type design.
2. Added some text in "terminals" section to mention serifs and clarify that serifs and terminals are not th same thing.
3. Added text for "Descenders" about how long descenders can be difficult to manage.
Merge pull request #27 from nikhilindia/patch-1

Update What_Is_a_Font.md