git workflow for making modifications you'll never push back to origin

Git newbie here.

I’m building an iPhone app with PhoneGap, which has a Git repos. I’d like to keep track of my changes to the PhoneGap code (mainly, adding files to the www directory) in my own Git repository, while being able to pull the latest and greatest out of PhoneGap.

What’s the best way of doing this? Is this what a branch is intended for?

Also, how would I get it to ignore the fact that I deleted folders I don’t need, like the android/ and blackberry/ folder?

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