Link to Examples page from README

@locochris for #22, I've created a [wiki page](https://github.com/rylnd/shpec/wiki/Examples) on which to add Examples. I'd love it if people used it to share how they're using shpec.

To that end, would you mind adding your `has_process_count` matcher from #17 to the wiki to get us started? I would've copied it from that PR but it looks like it's a little outdated.