FLINK-3529 Add template for pull requests

- link from CONTRIBUTING.md to PR template
- add it to excluded file of rat plugin

This closes #1729.

CONTRIBUTING.md

@@ -8,4 +8,4 @@ To make the process smooth for the project *committers* (those who review and ac
 
 Please check out the [How to Contribute guide](http://flink.apache.org/how-to-contribute.html) to understand how contributions are made. 
 A detailed explanation can be found in our [Contribute Code Guide](http://flink.apache.org/contribute-code.html) which also contains a list of coding guidelines that you should follow.
-
+For pull requests, there is a [check list](PULL_REQUEST_TEMPLATE.md) with criteria taken from the How to Contribute Guide and the Coding Guidelines.

PULL_REQUEST_TEMPLATE.md

+Thanks for contributing to Apache Flink. Before you open your pull request, please take the following check list into consideration.
+If your changes take all of the items into account, feel free to open your pull request. For more information and/or questions please refer to the [How To Contribute guide](http://flink.apache.org/how-to-contribute.html).
+In addition to going through the list, please provide a meaningful description of your changes.
+
+- [ ] General
+  - The pull request references the related JIRA issue
+  - The pull request addresses only one issue
+  - Each commit in the PR has a meaningful commit message
+
+- [ ] Documentation
+  - Documentation has been added for new functionality
+  - Old documentation affected by the pull request has been updated
+  - JavaDoc for public methods has been added
+
+- [ ] Tests & Build
+  - Functionality added by the pull request is covered by tests
+  - `mvn clean verify` has been executed successfully locally or a Travis build has passed

pom.xml

@@ -843,6 +843,7 @@ under the License.
 						<exclude>CONTRIBUTING.md</exclude>
 						<exclude>CONTRIBUTORS</exclude>
 						<exclude>DEPENDENCIES</exclude>
+						<exclude>PULL_REQUEST_TEMPLATE.md</exclude>
 						<!-- Build files -->
 						<exclude>**/*.iml</exclude>
 						<exclude>flink-quickstart/**/testArtifact/goal.txt</exclude>