Update & improve README

- Updated the list of rules in use (both imported and custom)
- Improved/less verbose wording
- Moved contribution guidelines to CONTRIBUTING.md

[ci skip]

Author: gmazzap

.github/CONTRIBUTING.md

@@ -0,0 +1,50 @@
+Thanks for contributing—you rock!
+
+# Getting Started
+
+* Make sure you have a [GitHub account](https://github.com/signup/free).
+* See if your issue has been discussed (or even fixed) earlier. You can [search for existing issues](../../../issues?q=is%3Aissue).
+* Assuming it does not already exist, [create a new issue](../../../issues/new).
+    * Clearly describe the issue. In case you want to report a bug, include steps to reproduce it.
+    * Make sure you fill in the earliest version that you know has the issue.
+* Fork the repository on GitHub.
+
+# Making Changes
+
+* Create a topic branch from where you want to base your work.
+    * Only target release branches if you are certain your fix must be on that branch.
+    * To quickly create a topic branch based on the `master` branch:
+        * `git checkout -b issue/%YOUR-ISSUE-NUMBER%_%DESCRIPTIVE-TITLE% version/2`
+        * a good example is `issue/123_typo_in_readme`
+* Make commits of logical units.
+* Make sure your commit messages are helpful.
+
+## Quality checks
+
+Before submitting a PR, please run code quality checks via:
+
+```shell
+$ composer qa
+```
+
+The command will run PHPCS and Psalm checks as well as PHPUnit tests.
+
+### Run specific unit tests
+
+Unit tests are organized in fixture files (see [`/tests/unit/fixtures`](https://github.com/inpsyde/php-coding-standards/tree/version/2/tests/unit/fixtures)).
+
+To run a single test use the `--filter` option:
+
+```shell
+$ composer tests -- --filter function-length-no-blank-lines
+```
+
+# Submitting Changes
+
+* Push your changes to the according topic branch in your fork of the repository.
+* [Create a pull request](../../../compare) to our repository.
+* Wait for feedback. The team looks at pull requests on a regular basis.
+
+# License
+
+By contributing code, you grant its use under the [MIT License](../LICENSE).

Inpsyde/ruleset.xml

@@ -110,13 +110,15 @@
 
     <!--
     PHPCS Extra
+    See: https://github.com/PHPCSStandards/PHPCSExtra
     -->
     <rule ref="NormalizedArrays.Arrays.CommaAfterLast" />
     <rule ref="Universal.Operators.StrictComparisons" />
     <rule ref="Universal.WhiteSpace.PrecisionAlignment" />
 
     <!--
     Slevomat
+    See https://github.com/slevomat/coding-standard
     -->
     <rule ref="SlevomatCodingStandard.Functions.ArrowFunctionDeclaration">
         <type>warning</type>