Enhancement: Add CONTRIBUTING.md

Author: localheinz

.github/CONTRIBUTING.md

@@ -0,0 +1,19 @@
+# CONTRIBUTING
+
+We are using [GitHub Actions](https://github.com/features/actions) as a continuous integration system.
+
+For details, take a look at the following workflow configuration file:
+
+- [`workflows/integrate.yaml`](workflows/integrate.yaml)
+
+## Tests
+
+We are using [`run-tests.php`](https://github.com/php/php-src/blob/f0c679c72ce02c1578ba9d56a099343b1eb3e16c/run-tests.php) to run [PHPT tests](https://qa.php.net/write-test.php).
+
+Run
+
+```
+php tests/run-tests.php
+```
+
+to run all the tests.

README.md

@@ -35,3 +35,7 @@ https://wiki.php.net/web/mirror
 
 Code must function on a vanilla PHP 7.3 installation. 
 Please keep this in mind before filing a pull request.
+
+## Contributing
+
+Please have a look at [`CONTRIBUTING.md`](.github/CONTRIBUTING.md).