Create pull request template (#70)

* Create pull request template

* add emoji

.github/pull_request_template.md

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+✋ A similar PR may already be submitted!
+Please search 🔎 among the [open pull requests][open-prs] before creating one.
+
+✋ If there isn't a similar PR, is there an [open issue][open-issues] associated 
+with what this PR is trying to solve?
+If not, let's create one before opening a PR. ✨
+
+Now that you've checked, it's time to create your PR. 📝
+Thanks for submitting! 🙏
+
+For more information, see the [contributing guide][contributing]. 👫
+
+**Summary**
+
+<!-- Summary of the PR -->
+
+Explain the **motivation** for making this change. What existing problem does the pull request solve? 🤔
+
+* [ ] 👯 This PR is not a duplicate
+* [ ] 📬 This PR addresses an open issue
+* [ ] ✅ This PR has passed CI
+
+[contributing]: https://github.com/rustwasm/book/blob/master/CONTRIBUTING.md
+[open-prs]: https://github.com/rustwasm/book/pulls
+[open-issues]: https://github.com/rustwasm/book/issues