[eclipse-iceoryx#2] Fill contributing.md file

CONTRIBUTING.md

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+# Contributing to Eclipse iceoryx2
+
+ 1. Every new contributor must sign the
+    [Eclipse Contributor Agreement (ECA)](https://www.eclipse.org/legal/ECA.php)
+    first.
+ 2. Before you start to work, please create an issue first.
+ 3. Create a branch with the prefix `iox2-$ISSUE_NUMBER$`.
+ 4. Every file requires this copyright header.
+
+    ```
+    // Copyright (c) 2023 Contributors to the Eclipse Foundation
+    //
+    // See the NOTICE file(s) distributed with this work for additional
+    // information regarding copyright ownership.
+    //
+    // This program and the accompanying materials are made available under the
+    // terms of the Apache Software License 2.0 which is available at
+    // https://www.apache.org/licenses/LICENSE-2.0, or the MIT license
+    // which is available at https://opensource.org/licenses/MIT.
+    //
+    // SPDX-License-Identifier: Apache-2.0 OR MIT
+    ```
+
+ 5. Every commit must have the prefix `[#$ISSUE_NUMBER$]`.
+ 6. When the work is done, please add your changes to the release notes in
+    `doc/release-notes/iceoryx2-unreleased.md`.
+ 7. Create a pull request.
+ 8. (optional) If you are a new contributor we would love to show our gratitude
+    for your support so please add yourself at the end of `README.md`.
+    The entry template looks like this:
+
+    ```html
+    <td align="center" valign="top" width="14.28%">
+    <a href="https://github.com/$USER_NAME$">
+    <img src="https://avatars.githubusercontent.com/u/$ID_OF_YOUR_PROFILE_PICTURE$" width="120px;" alt="$FIRST_NAME$ »$COOL_NICK_NAME$« $LAST_NAME$"/><br />
+    <sub><b>$FIRST_NAME$ »$COOL_NICK_NAME$« $LAST_NAME$</b></sub></a></td>
+    ```
+
+    **Notes:**
+     * The `$FIRST_NAME »$COOL_NICK_NAME$« $LAST_NAME$` is a suggestion but it can
+       be whatever you feel comfortable with.
+     * You can obtain the ID of your profile picture by clicking on your profile
+        icon on the top right, selecting `Your profile` and then right-clicking on
+        your profile picture and selecting `Copy image address`. The last number in
+        the URL is the ID of your profile picture.