mostly text refinements

CONTRIBUTING.md

@@ -3,7 +3,7 @@
 **Please read the following text before creating a pull request.**
 
 This project is organized and supported by community contributions and maintenance is done in our sparse time.
-We welcome any pull request that adds/extends functionality.
+We welcome any pull request that contributes to the PDFParser (code, documentation, ...).
 However, we would point out that you are initially responsible for a contribution.
 If you are new to pull requests see [Github documentation](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-pull-requests) for further information.
 Please don't just throw code at us and expect us to handle it.
@@ -16,7 +16,7 @@ The following points describe relevant preparations/inputs for the CI system.
 All checks must be green or a pull request is not accepted.
 
 * If you add/change functionality add at least **one test case** (unit test, system test, ...) to demonstrate that your code is working. There is no need to provide a full fledged PDF file to demonstrate a fix. Instead a unit test may be sufficient sometimes, have a look at [FontTest](https://github.com/smalot/pdfparser/blob/master/tests/PHPUnit/Unit/FontTest.php#L40) for example code.
-  * :exclamation: **If you dont know how to write tell us upfront when opening the pull request and we may add them ourselves or discuss other ways**. Code changes without tests are very likely to be rejected.
+  * :exclamation: **If you dont know how to write a test tell us upfront when opening the pull request and we may 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 point to start. Code changes without tests are very likely to be rejected.
 * Fix reported coding style issues. We use **PHP-CS-Fixer** for that. See https://github.com/smalot/pdfparser/blob/master/.php-cs-fixer.php for more information about our coding styles. See [Developer.md](./doc/Developer.md) for further information.
-* In case you **fix an existing issue**, refer to it in intro text of the pull request. In the following the correct Markdown syntax: `fixes #1234`. This example outlines that you are providing a fix for the issue `#1234`.
-* [ ] In case **you changed internal behavior/functionality** check our documentation to make sure these changes are **documented properly**: https://github.com/smalot/pdfparser/tree/master/doc
+* In case you **fix an existing issue**, refer to it in the intro text of the pull request. In the following the correct Markdown syntax: `fixes #1234`. This example outlines that you are providing a fix for the issue `#1234`.
+* In case **you changed internal behavior/functionality** check our documentation to make sure these changes are **documented properly**: https://github.com/smalot/pdfparser/tree/master/doc