Adding the text extraction of the PDF files found in data

Makefile

@@ -1,2 +1,6 @@
 help:
 	@cat doc/help.txt
+
+pdftotext:
+	@find ./data -iname '*.pdf' -execdir pdftotext {} \;
+	@find ./data -not \( -path ./data/text -prune \) -iname '*.txt' -exec mv {} './data/text/' ';'

doc/help.txt

@@ -1,2 +1,6 @@
 With this command, you'll be able to manage easily the extraction of
 URLs from books scanned by KBR
+
+Here is the list of commands and what there are doing:
+* make pdftotext: this command extract a text version of the PDF files
+                  and copy these files to the data/text/ directory