Add example for where to add require in Gatsby

[humphreybc]

Addresses https://github.com/KyleAMathews/typefaces/issues/41.

I was also unsure where to add the require statement, initially trying html.js with no luck.

[davelab6]

@KyleAMathews seems worth merging still

[zeekrey]

Would be cool if this could be merged. Or even extended by this:

41#issuecomment-387775041