Add pull request guidelines

.github/pull_request_template.md

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+If you're adding support for a new file type, please follow the below steps:
+
+- Add a fixture file named `fixture.<extension>` to the `fixture` directory.
+- Add the file extension to the `types` array in `test.js`.
+- Add the file type detection logic to the `index.js` file.
+- Add the file extension to the `Supported file types` section in the readme, in the format ```- [`<extension>`](URL) - Format name```, for example, ```- [`png`](https://en.wikipedia.org/wiki/Portable_Network_Graphics) - Portable Network Graphics```
+- Add the file extension to the `keywords` array in the `package.json` file.
+- Run `$ npm test` to ensure the tests pass.
+- Open a pull request with a little like `Add support for Format`, for example, `Add support for PNG`.
+- The pull request description should include a link to the official page of the file format or some other source. Also include a link to where you found the file type detection / magic bytes.