Link to real world libcnb.rs CNBs from docs/examples

[edmorley]

Whilst we have the examples under the examples/ directory, I think it would be helpful for users if we linked out to some of the real-world libcnb.rs-using CNBs from our docs. Either from the repo root README.md (from the same place that links to the examples), or else by adding an examples/README.md which could act as both an index for the examples and a way to link to external examples too.