fix: update nodejs readme with missing step (#1650)

Author: garysassano

nodejs/README.md

@@ -10,13 +10,21 @@ included and loaded automatically if you use the AWS SDK v2.
 
 ## Building
 
-To build the layer and sample applications, in this `nodejs` folder, run
+To build the layer and sample applications in this `nodejs` folder:
+
+First install dependencies:
 
 ```
 npm install
 ```
 
-This will download all dependencies and compile all code. The layer zip file will be present at `./packages/layer/build/layer.zip`.
+Then build the project:
+
+```
+npm run build
+```
+
+You'll find the generated layer zip file at `./packages/layer/build/layer.zip`.
 
 ## Sample applications