I would like to use MyGet build service to build my project hosted at GitHub. However, the service is triggered unnecessarily by updating README.md or other documents in the repository. Is there any way to skip these kinds of commits?
Unfortunately, there's no way out-of-the-box to filter commits, as there's no way for a machine to know what you'd like to build and what not (without going through extensive configuration). The GitHub commit webhook will fire either way, no matter what you commit. Any service responding to the webhook will listen for the event, including MyGet.
However, you could build your own triggers as MyGet Build Services supports POST web hooks. You could add your own filtering to your custom trigger, and choose when to fire the webhook, and when not to.
Details about how to create a custom build trigger for MyGet Build Services can be found here: http://docs.myget.org/docs/how-to/auto-trigger-a-myget-build-using-an-http-post-hook-url