add a PR template that links to the contribution guidelines (#3035)

Author: HackbrettXXX

.github/pull_request_template.md

@@ -0,0 +1,3 @@
+Thanks for contributing to jsPDF! Please follow our
+[Contribution Guidelines](https://github.com/MrRio/jsPDF/blob/master/CONTRIBUTING.md#pull-requests)
+when creating a pull request.

CONTRIBUTING.md

@@ -52,6 +52,7 @@ preparing a pull request please follow these guidelines:
   not babeled. When using newer EcmaScript or Browser APIs make sure the required polyfills are listed in
   `src/polyfills.js`.
 - Run `npm run prettier` before committing.
+- Don't update the files in `dist` in regular pull requests. These are usually only updated when creating a new release.
 - For the commit message, follow these guidelines:
   - Use the present tense ("Add feature" not "Added feature")
   - Use the imperative mood ("Move cursor to..." not "Moves cursor to...")