add docs for fileset and its example

This creates a new FileSet named \*lines\* that uses \`\`TextInputFormat\`\` and \`\`TextOutputFormat.\`\`
If you do not specify a base path, the dataset framework will generate a path
based on the dataset name. If you do not specify an input format, you will not be able
to use this as the input for a MapReduce Job; similarly, for the output format.