add .checkstyle file to apache-rat-plugin exclusion list

[reddycharan]

FEATURE REQUEST

1. Please describe the feature you are requesting.

eclipse/checkstyle plugin autogenerates .checkstyle file in all the projects/modules. So it has to be added to the exclusion list, otherwise apache-rat:check (mvn clean apache-rat:check install spotbugs:check) would fail because of these files.

2. Indicate the importance of this issue to you (blocker, must-have, should-have, nice-to-have). Are you currently using any workarounds to address this issue?

3. Provide any additional detail on your proposed use case for this feature.