Add info to the README about active C++ port and Ruby bindings

I forked the old deleted C++ code into it's own [light-weight repo](https://github.com/glassechidna/zxing-cpp), as I wanted to build some [regular Ruby (not JRuby) bindings](https://github.com/glassechidna/zxing_cpp.rb).

The Ruby bindings are actually just a fork of [Steven's Ruby bindings](https://github.com/smparkes/zxing.rb), where I've mostly just removed all the JRuby and MacRuby bits and then added all the stuff necessary to package and publish a [zxing_cpp gem](http://rubygems.org/gems/zxing_cpp).

Anyway, this commit just adds some info about the projects to the README so people looking for an active C++ port and some Ruby bindings can find them.