update contribute.html for regression testing

[Joe Buck]

> Someone also needs to look at the instructions in the info manual
> (gcc.texi) on sending patches (e.g. sending each logically separate patch
> separately), merge them into contribute.html and make the info manual
> point to contribute.html.

Yet another reason to do everything in texinfo and use texi2html to
generate the web pages.  Do we constantly want to be changing formats to
keep the manual and the web site in sync?