Add .npmignore to reduce NPM package size

[brianvanburken]

This will drastically reduce the size of the NPM package by removing unneeded files. It will keep the README.md. On my system the package size went from 68KB to 25KB.

[mcollina]

I do not see a reason for adding an .npmignore to the root folder. However, adding *.png to the assets it is worth doing: I checked and the png file account for all the saving. Would you mind amending your commit?

[brianvanburken]

@mcollina I've updated the PR to only ignore the .png file