Add a package-level doc string for the files package (ponylang#1640)

This commit adds a package-level doc string that describes the file
package and talks briefly about what the different actors, classes,
and primitives do. It also includes an example program to demonstrate
package works.

packages/files/files.pony

@@ -0,0 +1,47 @@
+"""
+# Files package
+
+The Files package provides classes for working with files and
+directories.
+
+Files are identified by `FilePath` objects, which represent both the
+path to the file and the capabilites for accessing the file at that
+path. `FilePath` objects can be used with the `CreateFile` and
+`OpenFile` primitives and the `File` class to get a reference to a
+file that can be used to write to and/or read from the file. It can
+also be used with the `Directory` object to get a reference to a
+directory object that can be used for directory operations.
+
+The `FileLine` class allows a file to be accessed one line at a time.
+
+The `FileStream` actor provides the ability to asynchronously write to
+a file.
+
+The `Path` primitive can be used to do path-related operations on
+strings and characters.
+
+# Example program
+
+This program opens the files that are given as command line arguments
+and prints their contents.
+
+```pony
+use "files"
+
+actor Main
+  new create(env: Env) =>
+    try
+      for file_name in env.args.slice(1).values() do
+        let path = FilePath(env.root as AmbientAuth, file_name)
+        match OpenFile(path)
+        | let file: File =>
+          while file.errno() is FileOK do
+            env.out.write(file.read(1024))
+          end
+        else
+          env.err.print("Error opening file '" + file_name + "'")
+        end
+      end
+    end
+```
+"""