Update guide with a link to resources to build required mobile icons and splash screens

Moving this over from https://github.com/meteor/meteor/issues/6069#issuecomment-200470236.

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(originally mentioned by @skirunman)

For Meteor Guide Mobile section it would be great if there was a link to resources to build all the required icons and splash screens. I have a pull request in for this package to generate the correct 1.3 resources. https://github.com/lpender/meteor-assets Would be nice to have this in the Guide or another supported option to generate all these assets.