Updated README to include linking instructions

Author: maneesht

README.md

@@ -168,6 +168,20 @@ implementation. The SDK is built via a combination of all of these packages
 which are published under the [`firebase`
 scope](https://www.npmjs.com/search?q=scope%3Afirebase) on NPM.
 
+### Testing the SDK Locally
+
+Please be sure to build your repo before proceeding any further.
+In order to manually test your SDK changes locally, you must use [yarn link](https://classic.yarnpkg.com/en/docs/cli/link):
+
+```shell
+$ cd packages/firebase
+$ yarn link # initialize the linking to the other folder
+$ cd ../<my-test-app-dir> # cd into your personal project directory
+$ yarn link firebase # tell yarn to use the locally built firebase SDK instead
+```
+
+This will create a symlink and point your `<my-test-app-dir>` to the locally built version of the firebase SDK.
+
 ### Helper Scripts
 
 Each package in the `packages` directory exposes a `dev` script. This script