Now it requires the datastore (or datastore.py plugin), if you want
to use runes. The simple reader/writer control lists don't require
that, but for simplicity we always require the runes Python module.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
1. Fix README.md dynamic plugin example (needs keywords, abspath!)
(pointed out by @carnhofdaki).
2. Define magic number we use (suggested by @FreePietje)
