You want to contribute to metadata-extractor? Great!
The easiest way to contribute is to provide test images for the image database.
Check the issues list for tasks that need attention. If you have something else in mind, that's great too. If you want to have your pull request merged, it's probably best to discuss the idea on the mailing list first.
There are a few simple but important guidelines for pull requests. Code not meeting these guidelines will need to be amended before being accepted.
Keep your commits short and sweet. Only change one thing at a time, and clearly identify the change in the commit message. See the recent project history for an idea of what this means.
Keep your PRs short and sweet. If you have several features you wish to contribute, split them out into separate PRs.
Match the existing code style. This include things like brace placement, indentation (spaces not tabs), and so on.
No 'churn'. If your IDE changes lots of code automatically, turn that feature off or use a more friendly IDE. If you think a wide-sweeping change should be applied to the codebase, please discuss that on the mailing list and, if agreed, it will be made in a single commit to all code.
Run the regression tests. The process is described here.
The goal of these guidelines is to make your contribution clearer to read and review for all, both now and in the future.
If you're new to GitHub's pull request model, this article might be helpful.