Update README.md

Mentioned of npm_modules in .dockerignore when using onbuild image

Author: Joan-Diego Rodriguez

README.md

@@ -42,6 +42,12 @@ The image assumes that your application has a file named
 dependencies and defining its [start
 script](https://docs.npmjs.com/misc/scripts#default-values).
 
+It also assumes that you have a file named [`.dockerignore`](https://docs.docker.com/engine/reference/builder/#/dockerignore-file) otherwise it will copy your local npm modules:
+
+```
+npm_modules
+```
+
 We have assembled a [Best Practices Guide](./docs/BestPractices.md) for those using these images on a daily basis.
 
 ## Run a single Node.js script