Added section on overriding Roller macros to the 2.x users guide on the Wiki

[Sean Gilligan]

I added a section on how to override/replace the use of velocimacro to 
the Wiki:
http://www.rollerweblogger.org/wiki/Wiki.jsp?page=UserGuide_2.x#ref-UserGuide_2.x-55
If I made any errors, please correct them.  If the new section is 
totally messed up or inappropriate, please delete it.  (But e-mail me 
and let me know why...)

Thanks,

Sean