CONTRIBUTING.md: small text refinement

CONTRIBUTING.md

@@ -9,11 +9,10 @@ If you are new to dealing with pull requests, you can find more information at [
 Please don't just throw code at us and expect us to handle it.
 Nevertheless, we will support you and give you feedback.
 
-## Important notes
-
 To make life easier for you and us, there is a Continuous Integration (CI) system that carries out software tests and performs a number of other tasks.
 The following points describe the relevant preparations/inputs for the CI system.
 All checks must be green, otherwise a pull request will not be accepted.
+* Please create an [issue](https://github.com/smalot/pdfparser/issues) before starting work on any significant changes.
 * We only accept code that is bundled with tests, regardless of whether it is a new function or a bug fix. This strengthens the code base and avoids later regressions. :exclamation: **If you don't know how to write a test, tell us upfront when you open the pull request and we might add them ourselves or discuss other ways**. This [Medium article](https://pguso.medium.com/a-beginners-guide-to-phpunit-writing-and-running-unit-tests-in-php-d0b23b96749f) might be a good starting point. Code changes without tests are very likely to be rejected.
 * Fix reported issues with the coding style. We use **PHP-CS-Fixer** for this. See [.php-cs-fixer.php](./.php-cs-fixer.php) for more information about our coding styles. [Developer.md](./doc/Developer.md) contains more information about this topic.
 * If you are fixing an **existing error**, refer to it in the introduction text of the pull request. For example, if you created a fix for issue `#1234` write the following Markdown: `fixes #1234`.