restructure readme

README.md

@@ -1,8 +1,12 @@
-== img2pdf ==
+img2pdf
+=======
 
 Lossless conversion of images to PDF without unnecessarily re-encoding JPEG
 files. Thus, no loss of quality and no unnecessary large output file.
 
+background
+----------
+
 PDF is able to embed JPEG images as they are without re-encoding them (and
 hence loosing quality) but I was missing a tool to do this automatically, thus
 I wrote this piece of python code.
@@ -11,6 +15,9 @@ If you know how to embed JPEG images into a PDF container without
 recompression, using existing tools, please contact me so that I can put this
 code into the garbage bin :D
 
+functionality
+-------------
+
 The program will take image filenames from commandline arguments and output a
 PDF file with them embedded into it. If the input image is a JPEG file, it will
 be included as-is without any processing. If it is in any other format, the
@@ -33,6 +40,9 @@ image is given, losslessly embed JPEGs into a PDF container without adding more
 overhead than the PDF structure itself and will save all other graphics formats
 using lossless zip-compression.
 
+bugs
+----
+
 If you find a JPEG that, when embedded can not be read by the Adobe Acrobat
 Reader, please contact me.