doc: note how to do Rust builds on Ubuntu, at least.

In particular, using cargo to install rustfmt doesn't work.

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

doc/INSTALL.md

@@ -61,6 +61,10 @@ For development or running tests, get additional dependencies:
     sudo apt-get install -y valgrind libpq-dev shellcheck cppcheck \
       libsecp256k1-dev jq
 
+If you want to build the Rust plugins (currently, cln-grpc):
+
+	sudo apt-get install -y cargo rustfmt
+
 Build lightning:
 
     poetry install
@@ -74,8 +78,6 @@ Running lightning:
     ./lightningd/lightningd &
     ./cli/lightning-cli help
 
-**Note**: You may need to include `testnet=1` in `bitcoin.conf`
-
 To Build on Fedora
 ---------------------