[Contributing] Broken link

[dourouc05]

In http://www.juliafem.org/contributing/, the link "how to make changes to an existing packages" is broken (first paragraph). I don't know the new place for this section, though :(.

[ahojukka5]

Well it's here, but the documentation looks to be for Julia v0.6, thus obsolete information.

[ahojukka5]

We should rewrite this to match Julia 1.0, things have changed a lot lately.