Merge pull request #4390 from bhandras/etcd_docs

docs: documentation for the experimental etcd feature

docs/etcd.md

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+# Experimental etcd support in LND
+
+With the recent introduction of the `kvdb` interface LND can support multiple
+database backends allowing experimentation with the storage model as well as
+improving robustness trough eg. replicating essential data.
+
+Building on `kvdb` in v0.11.0 we're adding experimental [etcd](https://etcd.io)
+support to LND. As this is an unstable feature heavily in development, it still
+has *many* rough edges for the time being. It is therefore highly recommended to
+not use LND on `etcd` in any kind of production environment especially not
+on bitcoin mainnet.
+
+## Building LND with etcd support
+
+To create a dev build of LND with etcd support use the following command:
+
+```
+make tags="kvdb_etcd"
+```
+
+The important tag is the `kvdb_etcd`, without which the binary is built without
+the etcd driver.
+
+For development it is advised to set the `GOFLAGS` environment variable to 
+`"-tags=test"` otherwise `gopls` won't work on code in `channeldb/kvdb/etcd`
+directory.
+
+## Running a local etcd instance for testing
+
+To start your local etcd instance for testing run:
+
+```
+./etcd \
+    --auto-tls \
+    --advertise-client-urls=https://127.0.0.1:2379 \
+    --listen-client-urls=https://0.0.0.0:2379 \
+    --max-txn-ops=16384 \
+    --max-request-bytes=104857600
+```
+
+The large `max-txn-ops` and `max-request-bytes` values are currently required in
+case of running LND with the full graph in etcd. Upcoming versions will split
+the database to local and replicated parts and only essential parts will remain
+in the replicated database, removing the requirement for these additional 
+settings. These parameters have been tested to work with testnet LND.
+
+## Configuring LND to run on etcd
+
+To run LND with etcd, additional configuration is needed, specified either
+through command line flags or in `lnd.conf`.
+
+Sample command line:
+
+```
+./lnd-debug \
+    --db.backend=etcd \
+    --db.etcd.host=127.0.0.1:2379 \
+    --db.etcd.certfile=/home/user/etcd/bin/default.etcd/fixtures/client/cert.pem \
+    --db.etcd.keyfile=/home/user/etcd/bin/default.etcd/fixtures/client/key.pem \
+    --db.etcd.insecure_skip_verify
+```
+
+Sample `lnd.conf` (with other setting omitted):
+
+```
+[db]
+backend=etcd
+etcd.host=127.0.0.1:2379
+etcd.cerfile=/home/user/etcd/bin/default.etcd/fixtures/client/cert.pem
+etcd.keyfile=/home/user/etcd/bin/default.etcd/fixtures/client/key.pem
+etcd.insecure_skip_verify=true
+```
+
+Optionally users can specifiy `db.etcd.user` and `db.etcd.pass` for db user
+authentication.
+
+## Migrating existing channel.db to etcd
+
+This is currently not supported.
+
+## Disclaimer
+
+As mentioned before this is an experimental feature, and with that your data
+may be lost. Use at your own risk!