adding pull-request template

PULL_REQUEST_TEMPLATE.md

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+**Yes, I promise I've read the [Contributions Guidelines](https://github.com/getify/You-Dont-Know-JS/blob/master/CONTRIBUTING.md)** (please feel free to remove this line).
+
+Specifically quoting these guidelines regarding typos:
+
+> Typos?
+>
+> Please don't worry about minor text typos. These will almost certainly be caught during the editing process.
+>
+> If you're going to submit a PR for typo fixes, please be measured in doing so by collecting several small changes into a single PR (in separate commits). Or, **just don't even worry about them for now,** because we'll get to them later. I promise.
+
+----
+
+**Please type "I already searched for this issue":**
+
+**Edition:** (pull requests not accepted for previous editions)
+
+**Book Title:**
+
+**Chapter:**
+
+**Section Title:**
+
+**Topic:**