added instructions to run web sample apps (#78)

Updated readme to include instructions to be able to build web sample apps locally.

web/readme.md

@@ -1 +1,14 @@
 Samples for [Flutter for web](https://flutter.dev/web).
+
+#### Building Samples code
+Set up `webdev` for first time as below:
+```
+$ pub global activate webdev
+$ pub get
+```
+
+Build and run code locally at `http://localhost:8080`
+```
+$ webdev serve
+```
+Checkout [this link](https://dart.dev/tools/webdev#using-webdev-and-build_runner-commands) for more details about building, serving and testing code.