[build] Move GitHub specific files to '.github', update release script

Author: StephanEwen

CONTRIBUTING.md renamed to .github/CONTRIBUTING.md

@@ -931,11 +931,7 @@ under the License.
 						<exclude>**/flink-bin/conf/zoo.cfg</exclude>
 						<!-- Administrative files in the main trunk. -->
 						<exclude>**/README.md</exclude>
-						<exclude>CHANGELOG</exclude>
-						<exclude>CONTRIBUTING.md</exclude>
-						<exclude>CONTRIBUTORS</exclude>
-						<exclude>DEPENDENCIES</exclude>
-						<exclude>PULL_REQUEST_TEMPLATE.md</exclude>
+						<exclude>.github/**</exclude>
 						<!-- Build files -->
 						<exclude>**/*.iml</exclude>
 						<exclude>flink-quickstart/**/testArtifact/goal.txt</exclude>

PULL_REQUEST_TEMPLATE.md renamed to .github/PULL_REQUEST_TEMPLATE.md

@@ -93,9 +93,11 @@ prepare() {
   cd flink
   git checkout -b "release-$RELEASE_VERSION-$RELEASE_CANDIDATE" origin/$RELEASE_BRANCH
   rm -f .gitignore
+  rm -f .gitattributes
   rm -f .travis.yml
   rm -f deploysettings.xml
   rm -f CHANGELOG
+  rm -rf .github
   cd ..
 }