README.md: additional noncore platforms

A problem that prevents `node` releases getting packaged for debian and ubuntu is the need to compile on all required debian platforms.  Currently new versions of `node` are breaking on MIPS and GNU hurd, mainly due to minor libuv problems (eg missing headers, undefined constants).

Aiming to at least test those platforms would help keep fresh `io.js` versions coming to debian/ubuntu users through "official" channels.