Add CONTRIBUTING docs to DWDS (#1471)

Author: elliette

dwds/CONTRIBUTING.md

@@ -0,0 +1,21 @@
+## Local Development
+
+1. In the `/webdev` directory, run `pub global activate --source path webdev` (this only needs to be run once)
+2. Uncomment the dwds dependency override in `/webdev/webdev/pubspec.yaml`, then run `dart run build_runner build` from `/webdev/webdev` directory
+    * *Note: You will have to comment and build, and then uncomment and build, each time you need to pick up new changes*
+2. From `/webdev/example`, run `webdev serve --debug --verbose` (Note: all options can be found by running `webdev help serve`)
+3. Type opt/alt-d in the browser. This is required to start the VM.
+4. [OPTIONAL] If you need to connect a locally running DevTools (instructions for running [here](https://github.com/flutter/devtools/blob/master/CONTRIBUTING.md )), then close the DevTools that automatically opened in the previous step. Copy and paste the debug URI (should be logged in your terminal) into the DevTools connection box.
+
+### Note:
+If you get this error:
+
+`The /webdev/webdev/pubspec.yaml file has changed since the /webdev/webdev/pubspec.lock file was generated, please run "dart pub get" again.` 
+
+You need to do the following:
+
+* `rm webdev/webdev/pubspec.lock`
+* Then, from `/webdev/webdev` run `dart pub get`
+
+## Release steps 
+TODO