docs: More readme

README.md

@@ -13,9 +13,29 @@ Get a dict-like object to list and read the pdfs of a folder, as text:
     >>> from pdfdol import PdfFilesReader
     >>> from pdfdol.tests import get_test_pdf_folder
     >>> folder_path = get_test_pdf_folder()
-    >>> s = PdfFilesReader(folder_path)
-    >>> sorted(s)
+    >>> pdfs = PdfFilesReader(folder_path)
+    >>> sorted(pdfs)
     ['sample_pdf_1', 'sample_pdf_2']
-    >>> assert s['sample_pdf_2'] == [
+    >>> assert pdfs['sample_pdf_2'] == [
     ...     'Page 1\nThis is a sample text for testing Python PDF tools.'
     ... ]
+
+See that the values of a `PdfFilesReader` are lists of pages. 
+If you need strings (i.e. all the pages together) you can add a decoder like so:
+
+```python
+from dol import add_decoder
+page_separator = '---------------------'
+pdfs = add_decoder(pdfs, decoder=page_separator.join)
+```
+
+If you need this at the level of the class, just do this:
+
+```python
+from dol import add_decoder
+page_separator = '---------------------'
+FilesReader = add_decoder(PdfFilesReader, decoder=page_separator.join)
+# and then
+pdfs = FilesReader(folder_path)
+# ...
+```