Issue #225 Improve docs on how to add files with password

Author: mikehaertl

README.md

@@ -51,14 +51,42 @@ composer require mikehaertl/php-pdftk
 
 ## Examples
 
+### Create instance for PDF files
+
+There are several ways to tell the `Pdf` instance which file(s) it should use.
+Some files may also require a password or need an alias to be used as a handle
+in some operations (e.g. cat or shuffle).
+
+> **Note:** In version 2.x of pdftk a handle can be one or more upper case letters.
+
+```php
+// Create an instance for a single file
+$pdf = new Pdf('/path/to/form.pdf');
+
+// Alternatively add files later. Handles are autogenerated in this case.
+$pdf = new Pdf();
+$pdf->addFile('/path/to/file1.pdf');
+$pdf->addFile('/path/to/file2.pdf');
+
+// Add files with own handle
+$pdf = new Pdf();
+$pdf->addFile('/path/to/file1.pdf', 'A');
+$pdf->addFile('/path/to/file2.pdf', 'B');
+// Add file with handle and password
+$pdf->addFile('/path/to/file3.pdf', 'C', 'secret*password');
+
+// Shortcut to pass all files to the constructor
+$pdf = new Pdf([
+  'A' => ['/path/to/file1.pdf', 'secret*password1'],
+  'B' => ['/path/to/file2.pdf', 'secret*password2'],
+]);
+```
+
 ### Operations
 
 Please consult the `pdftk` man page for each operation to find out how each operation works
 in detail and which options are available.
 
-> **Note:** Some commands allow to alias your files with a handle (see examples below).
-> In version 2.x of pdftk a handle can be one or more upper case letters.
-
 For all operations you can either save the PDF locally through `saveAs($name)` or send it to the
 browser with `send()`. If you pass a filename to `send($name)` the client browser will open a download
 dialogue whereas without a filename it will usually display the PDF inline.
@@ -132,12 +160,12 @@ if ($result === false) {
     $error = $pdf->getError();
 }
 
-// Combine pages from several files, demonstrating several ways how to add files
+// Combine pages from several files
 $pdf = new Pdf([
     'A' => '/path/file1.pdf',                 // A is alias for file1.pdf
     'B' => ['/path/file2.pdf','pass**word'],  // B is alias for file2.pdf
+    'C' => ['/path/file3.pdf','secret**pw'],  // C is alias for file3.pdf
 ]);
-$pdf->addFile('/path/file3.pdf','C','**secret**pw');  // C is alias file3.pdf
 $result = $pdf->cat(1, 5, 'A')                // pages 1-5 from A
     ->cat(3, null, 'B')             // page 3 from B
     ->cat(7, 'end', 'B', null, 'east') // pages 7-end from B, rotated East