Add contributing info (apache#6771)

Motivation
It is a common practice to have the CONTRIBUTING.md file with info on how to start contributing to the project.

Modifications
CONTRIBUTING.md was added to the project with the link to the official Apache Pulsar page.

CONTRIBUTING.md

@@ -0,0 +1,26 @@
+<!--
+
+    Licensed to the Apache Software Foundation (ASF) under one
+    or more contributor license agreements.  See the NOTICE file
+    distributed with this work for additional information
+    regarding copyright ownership.  The ASF licenses this file
+    to you under the Apache License, Version 2.0 (the
+    "License"); you may not use this file except in compliance
+    with the License.  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing,
+    software distributed under the License is distributed on an
+    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+    KIND, either express or implied.  See the License for the
+    specific language governing permissions and limitations
+    under the License.
+
+-->
+
+## Contributing to Apache Pulsar
+
+We would love for you to contribute to Apache Pulsar and make it even better!
+Please check the [Contributing to Apache Pulsar](https://pulsar.apache.org/en/contributing/) 
+page before starting to work on the project.