Update from-source install instruction in README (helix-editor#2603)

README.md

@@ -40,11 +40,11 @@ If you would like to build from source:
 git clone https://github.com/helix-editor/helix
 cd helix
 cargo install --path helix-term
-hx --grammar fetch
-hx --grammar build
 ```
 
 This will install the `hx` binary to `$HOME/.cargo/bin` and build tree-sitter grammars.
+If you want to customize your `languages.toml` config,
+tree-sitter grammars may be manually fetched and built with `hx --grammar fetch` and `hx --grammar build`.
 
 Helix also needs its runtime files so make sure to copy/symlink the `runtime/` directory into the
 config directory (for example `~/.config/helix/runtime` on Linux/macOS, or `%AppData%/helix/runtime` on Windows).