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

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HDFS-4980. Merge 1502302 from trunk

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HDFS-4980. Missed change to CHANGES.txt from previous commit 1502308

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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