docs: add sqlite instructions

docs/INSTALL.md

@@ -254,6 +254,7 @@ will have the following tags:
 - [monitoring](/monitoring) (for Prometheus integration)
 - [peersrpc](/lnrpc/peersrpc/peers.proto)
 - [kvdb_postrgres](/docs/postgres.md)
+- [kvdb_sqlite](/docs/sqlite.md)
 - [kvdb_etcd](/docs/etcd.md)
 
 The `dev` tag is used for development builds, and is not included in the

docs/sqlite.md

@@ -0,0 +1,72 @@
+# SQLite support in LND
+
+With the introduction of the `kvdb` interface, LND can support multiple database
+backends. One of the supported backends is 
+[sqlite](https://www.sqlite.org/index.html). This document describes how use 
+LND with a sqlite backend.
+
+Note that for the time being, the sqlite backend option can only be set for 
+_new_ nodes. Setting the option for an existing node will not migrate the data. 
+
+## Supported platforms and architectures
+
+Note that the sqlite backend is _not_ supported for Windows (386/ARM) or for 
+Linux (PPC/MIPS) due to these platforms [not being supported by the sqlite 
+driver library.](
+https://pkg.go.dev/modernc.org/sqlite#hdr-Supported_platforms_and_architectures)
+
+## Configuring LND for SQLite
+
+LND is configured for SQLite through the following configuration options:
+
+* `db.backend=sqlite` to select the SQLite backend.
+* `db.sqlite.timeout=...` to set the connection timeout. If not set, no
+  timeout applies.
+* `db.sqlite.busytimeout=...` to set the maximum amount of time that a db call 
+  should wait if the db is currently locked.
+* `db.sqlite.pragmaoptions=...` to set a list of pragma options to be applied 
+  to the db connection. See the 
+  [sqlite documentation](https://www.sqlite.org/pragma.html) for more 
+  information on the available pragma options.
+
+## Default pragma options
+
+Currently, the following pragma options are always set:
+
+```
+foreign_keys=on
+journal_mode=wal
+busy_timeout=5000 // Overried with the db.sqlite.busytimeout option.
+```
+
+The following pragma options are set by default but can be overridden using the
+`db.sqlite.pragmaoptions` option:
+
+``` 
+synchronous=full
+auto_vacuum=incremental
+fullfsync=true // Only meaningful on a Mac. 
+```
+
+## Auto-compaction
+
+To activate auto-compaction on startup, the `incremental_vacuum` pragma option
+should be used:
+```
+// Use N to restrict the maximum number of pages to be removed from the 
+// freelist.
+db.sqlite.pragmaoptions=incremental_vacuum(N) 
+
+// Omit N if the entire freelist should be cleared.
+db.sqlite.pragmaoptions=incremental_vacuum
+```
+
+Example as follows:
+```
+[db]
+db.backend=sqlite
+db.sqlite.timeout=0
+db.sqlite.busytimeout=10s
+db.sqlite.pragmaoptions=temp_store=memory
+db.sqlite.pragmaoptions=incremental_vacuum
+```