Alternative list of public grlc instances in README

[albertmeronyo]

[c-martinez]

Should this be in the README? Or on a sub-page (wiki) linked from the README?

[tkuhn]

Now that my issue #359 is resolved, I can offer this grlc instance for public use (no warranties): http://grlcplain.nanopubs.lod.labs.vu.nl/