From what I have learnt this is the format
.info file
----------
1st line - Filename of archive
2nd line - Program and version
3rd line - OS Version (I think, currently R3-01) and packaging date in brackets (in YYYYMMDD format)
4th line - optional Comment line can be left out (I believe)
5th line - number of files and packaging date (in YYYY-MM-DD format)
6th line (onwards) - Size and name of files
The .patch or .xap file is a tar.gz of the patch.
An example of one my info files I am working on is:
libSDL_ttf-2.0.9-R3-01(20080920).xap
libSDL_ttf-2.0.9
R3-01(20080920)
I made this....
12:2008-09-20
12K include/SDL/SDL_ttf.h
16K include/SDL
20K include
4.0K lib/libSDL_ttf.la
0 lib/libSDL_ttf.so
96K lib/libSDL_ttf.a
0 lib/libSDL_ttf-2.0.so.0
84K lib/libSDL_ttf-2.0.so.0.6.3
188K lib
0 share/info/libSDL_ttf-2.0.9-R3-01(20080920).info
4.0K share/info
8.0K share
A script to help you get started producing them is
http://www.xs4all.nl/~jostht/epc/ called mkxap.