doc: document the plugin interface to gather Bitcoin data

Changelog-Added: plugins: the interface to gather data from Bitcoin backend plugins is now final.

doc/PLUGINS.md

@@ -939,6 +939,81 @@ This will ensure backward
 compatibility should the semantics be changed in future.
 
 
+
+## Bitcoin backend
+
+C-lightning communicates with the Bitcoin network through a plugin. It uses the
+`bcli` plugin by default but you can use a custom one, multiple custom ones for
+different operations, or write your own for your favourite Bitcoin data source!
+
+Communication with the plugin is done through 5 JSONRPC commands, `lightningd`
+can use from 1 to 5 plugin(s) registering these 5 commands for gathering Bitcoin
+data. Each plugin must follow the below specification for `lightningd` to operate.
+
+
+### `getchainfo`
+
+Called at startup, it's used to check the network `lightningd` is operating on and to
+get the sync status of the backend.
+
+The plugin must respond to `getchainfo` with the following fields:
+    - `chain` (string), the network name as introduced in bip70
+    - `headercount` (number), the number of fetched block headers
+    - `blockcount` (number), the number of fetched block body
+    - `ibd` (bool), whether the backend is performing initial block download
+
+
+### `estimatefees`
+
+Polled by `lightningd` to get the current feerate, all values must be passed in BTC/kVB.
+
+If fee estimation fails, the plugin must set all the fields to `null`.
+
+The plugin, if fee estimation succeeds, must respond with the following fields:
+    - `opening` (number), used for funding and also misc transactions
+    - `mutual_close` (number), used for the mutual close transaction
+    - `unilateral_close` (number), used for unilateral close (/commitment) transactions
+    - `delayed_to_us` (number), used for resolving our output from our unilateral close
+    - `htlc_resolution` (number), used for resolving HTLCs after an unilateral close
+    - `penalty` (number), used for resolving revoked transactions
+    - `min` (number), used as the minimum acceptable feerate
+    - `max` (number), used as the maximum acceptable feerate
+
+
+### `getrawblockbyheight`
+
+This call takes one parameter, `height`, which determines the block height of
+the block to fetch.
+
+The plugin must set all fields to `null` if no block was found at the specified `height`.
+
+The plugin must respond to `getrawblockbyheight` with the following fields:
+    - `blockhash` (string), the block hash as a hexadecimal string
+    - `block` (string), the block content as a hexadecimal string
+
+
+### `getutxout`
+
+This call takes two parameter, the `txid` (string) and the `vout` (number)
+identifying the UTXO we're interested in.
+
+The plugin must set both fields to `null` if the specified TXO was spent.
+
+The plugin must respond to `gettxout` with the following fields:
+    - `amount` (number), the output value in **sats**
+    - `script` (string), the output scriptPubKey
+
+
+### `sendrawtransaction`
+
+This call takes one parameter, a string representing a hex-encoded Bitcoin
+transaction.
+
+The plugin must broadcast it and respond with the following fields:
+    - `succes` (boolean), which is `true` if the broadcast succeeded
+    - `errmsg` (string), if success is `false`, the reason why it failed
+
+
 [jsonrpc-spec]: https://www.jsonrpc.org/specification
 [jsonrpc-notification-spec]: https://www.jsonrpc.org/specification#notification
 [bolt4]: https://github.com/lightningnetwork/lightning-rfc/blob/master/04-onion-routing.md