sauron: add a basic README

sauron/README.md

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+## Sauron
+
+A Bitcoin backend plugin relying on [Esplora](https://github.com/Blockstream/esplora).
+
+
+### About
+
+It allows C-lightning to run without needing a *local* `bitcoind`, and can be either
+self-hosted (Esplora is Open Source, and self hosting it is basically a `docker` one-liner).
+
+This is still a WIP, so is the API C-lightning side. So not to be used for real.
+
+
+### Run
+
+You need to:
+- disable the default Bitcoin backend (`bcli`)
+- register sauron
+- provide the API endpoint you want to use
+
+Here is a fully reptilian example running against [blockstream.info](https://blockstream.info/):
+
+```
+lightningd --mainnet --disable-plugin bcli --plugin $PWD/sauron.py --sauron-api-endpoint https://blockstream.info/api/
+```