Add a few more elements introduced by HTML5
It's apparently spelled 'semantic':grey_exclamation:
Clarify where to drag files.

In the class I TAed, students were dragging the `index.html` file
directly onto the already open text editor window, which simply
inserted the contents of the file into their `hello.html`. This
was pretty confusing to them, so I had to teach them about dragging
onto the text editor icon instead of the window that was open.
Merge branch 'icon-drag' of https://github.com/indirect/docs into indirect-icon-drag
Merge pull request #215 from AimeeKnight/master

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