Adds an explanatory README.md

When accepted, closes jrockway#2

README.md

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+# MooseX::FileAttribute
+
+This is the repository for
+the
+[CPAN module `File::Attribute`](https://metacpan.org/release/MooseX-FileAttribute). It
+is part of the [Moose](https://metacpan.org/pod/Moose) set of plugins,
+and it allows to easily deal with instance variables that contain file
+or directory paths, adding the `has_file` and `has_directory`
+attributes to the Moose set of variables.