fix index for new section on Ethereum

Author: ma2bd

src/SUMMARY.md

@@ -61,6 +61,8 @@
 
   - [Machine Learning](developers/experimental/ml.md)
 
+  - [Ethereum](developers/experimental/ethereum.md)
+
 ---
 
 # Node Operators

src/developers/experimental.md

@@ -1,5 +1,11 @@
 # Experimental Topics
 
-In this section, we present experimental topics related to the Linera protocol.
+In this section, we present some experimental features of the Linera protocol.
 
-These are still in the works and subject to frequent breaking changes.
+Experimental features are generally not available on the Linera testnet. You may
+have to compile Linera from sources from the `main` branch of
+[the repository](https://github.com/linera-io/linera-protocol), then run a
+[local test network](getting_started/hello_linera.md#using-a-local-test-network).
+
+You may also need to select specific Cargo features during compilation using
+`--features`.