added instructions to build the book

It is required to make the images folder if it does not exists yet
and one wants to build the book with the asciidoctor command.
closes #764

Author: aollier

README.asc

@@ -27,9 +27,19 @@ Converting to EPub...
 Converting to Mobi (kf8)...
  -- Mobi output at progit.mobi
 Converting to PDF...
- -- PDF  output at progit.pdf
+ -- PDF output at progit.pdf
 ----
 
+An alternative to calling the `bundle` command is to call directly the `asciidoctor` command.
+If it is the first time you build the book, it is necessary to create the _images_ folder and fill it.
+Do the following only once:
+
+----
+$ mkdir images/
+$ find book/*/images/ -type f -and -name '*.pnd' -exec cp '{}' images/ \;
+----
+
+Then you can build the book.
 You can also use the following commands:
 
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