HADOOP-9432 Add support for markdown .md files in site documentation

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HDFS-4980. Merge 1502302 from trunk

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1502308 13f79535-47bb-0310-9956-ffa450edef68
HDFS-4980. Missed change to CHANGES.txt from previous commit 1502308

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1502311 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-9417.  Support for symlink resolution in LocalFileSystem / RawLocalFileSystem.  (Andrew Wang via Colin Patrick McCabe)

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Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-HADOOP-9166-branch-2-gd