Created pull request template

pull_request_template.md

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+Thanks for making a contribution to the CS224W course notes! The staff and future students appreciate it. Make sure you've published your changes as a live preview. The URL will look like `https://<your github id>.github.io/cs224w-notes/`. Provide the link to the live preview in your pull request. Also please make sure there are no content, spelling, or grammar mistakes in your pull request before submitting it for review. Strive to make your notes as complete, correct, and as easy to understand as possible. Pull requests that have mistakes, and require many rounds of review before acceptance, will likely receive less extra credit.
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+Please delete this template and replace it with your own pull request message.