Update README.md (#571)

* Update README.md

The `yarn build-npm` step will fail without typescript types installed. This PR adds a step to install them.

* Update README.md

Co-authored-by: Ping Yu <[email protected]>

Author: mjmaurer

deeplab/demo/README.md

@@ -4,6 +4,8 @@ This demo allows you to try out semantic segmentation on a couple of preset imag
 
 ## Setup
 
+Run `yarn` in the root and 'deeplab' folder to install dependencies.
+
 Run `yarn build-npm` in the 'deeplab' folder.
 
 Change the directory to the `demo` folder: