Add Electron logo icon asset

[mnquintana]

It would be really helpful to have an SVG of just the Electron logo icon (ie. no text) - this PR could really use it, since Nodegit's website is currently using Atom's logo when it should be the Electron logo.

/cc @simurai @jlord

[kevinsawicki]

@simurai would you mind adding this to this repo? Maybe just call it electron-icon.svg and throw it in images/? What do you think?

[simurai]

Sure.. added: https://github.com/atom/electron.atom.io/tree/gh-pages/images

Also, as a PNG in case.

[kevinsawicki]

Awesome, thanks!