CONTRIBUTE.md: Fix link to contribution guidelines
CONTRIBUTE.md: Fix link to contribution guidelines

The old link was to a wiki that seems to no longer exist. In README.md,
I found a link to the contribution guidelines on readthedocs, so I
assume that's the one we want to use.

AnnikaH/IncludeOS,hioa-cs/IncludeOS,mnordsletten/IncludeOS,alfred-bratterud/IncludeOS,hioa-cs/IncludeOS,hioa-cs/IncludeOS,AnnikaH/IncludeOS,AndreasAakesson/IncludeOS,alfred-bratterud/IncludeOS,hioa-cs/IncludeOS,AndreasAakesson/IncludeOS,AndreasAakesson/IncludeOS,AnnikaH/IncludeOS,mnordsletten/IncludeOS,mnordsletten/IncludeOS,AndreasAakesson/IncludeOS,alfred-bratterud/IncludeOS,AndreasAakesson/IncludeOS,AnnikaH/IncludeOS,mnordsletten/IncludeOS,AndreasAakesson/IncludeOS,mnordsletten/IncludeOS,alfred-bratterud/IncludeOS,AnnikaH/IncludeOS,mnordsletten/IncludeOS,alfred-bratterud/IncludeOS,hioa-cs/IncludeOS