Add missing javadoc tags

src/main/java/net/kemitix/files/FileReader.java

@@ -11,6 +11,7 @@ public interface FileReader {
      * be replaced with a newline (\n).</p>
      *
      * @param file the file to read
+     * @return the contents of the file
      * @throws IOException if there is an error.
      */
     String read(File file) throws IOException;

src/main/java/net/kemitix/files/FileWriter.java

@@ -13,6 +13,7 @@ public interface FileWriter {
      *
      * @param file    the file to write
      * @param content the content to write
+     * @throws IOException if there is an error.
      */
     void write(
             File file,