doc: add a troubleshooting note

docs/next/troubleshooting.md

@@ -18,3 +18,18 @@ console_error_panic_hook::set_once();
 ## Debugging using DWARF + WASM
 
 > Note: This section is a stub. Help us write this section!
+
+## unexpected `cfg` condition name: `sycamore_force_ssr`
+
+Sycamore uses a custom cfg (`sycamore_force_ssr`) to force the SSR mode. Because
+the compiler doesn't know about custom cfg it will emit warnings. To disables
+those warnings, add the following lints configuration in the `Cargo.toml` file
+of your project.
+
+```toml
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ["cfg(sycamore_force_ssr)"] }
+```
+
+More information here:
+<https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html#check-cfg-in-lintsrust-table>