doc: document that we can build lowdown, remove from Alpine.

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

doc/INSTALL.md

@@ -65,6 +65,8 @@ For development or running tests, get additional dependencies:
     sudo apt-get install -y valgrind libpq-dev shellcheck cppcheck \
       libsecp256k1-dev jq
 
+If you can't install `lowdown`, a version will be built in-tree.
+
 If you want to build the Rust plugins (currently, cln-grpc):
 
 	sudo apt-get install -y cargo rustfmt