Indicate how to get set up for the multitenancy integration tests. (#393)

This documentation is based off of the instructions in https://github.com/firebase/firebase-admin-node/blob/master/CONTRIBUTING.md.

Author: micahstairs

CONTRIBUTING.md

@@ -141,8 +141,17 @@ Authentication Admin` role at
 [Google Cloud Platform Console / IAM & admin](https://console.cloud.google.com/iam-admin). This is
 required to ensure that exported user records contain the password hashes of the user accounts.
 Also obtain the web API key of the project from the "Settings > General" page, and save it as
-`integration_apikey.txt` at the root of the codebase. Now run the following command to invoke the
-integration test suite:
+`integration_apikey.txt` at the root of the codebase.
+
+Some of the integration tests require an
+[Identity Platform](https://cloud.google.com/identity-platform/) project with multi-tenancy
+[enabled](https://cloud.google.com/identity-platform/docs/multi-tenancy-quickstart#enabling_multi-tenancy).
+An existing Firebase project can be upgraded to an Identity Platform project without losing any
+functionality via the
+[Identity Platform Marketplace Page](https://console.cloud.google.com/customer-identity). Note that
+charges may be incurred for active users beyond the Identity Platform free tier.
+
+Now run the following command to invoke the integration test suite:
 
 ```
 mvn verify
@@ -153,14 +162,14 @@ tests, specify the `-DskipUTs` flag.
 
 ### Generating API Docs
 
-Invoke the [Maven Javadoc plugin](https://maven.apache.org/plugins/maven-javadoc-plugin/) as 
+Invoke the [Maven Javadoc plugin](https://maven.apache.org/plugins/maven-javadoc-plugin/) as
 follows to generate API docs for all packages in the codebase:
 
 ```
 mvn javadoc:javadoc
 ```
 
-This will generate the API docs, and place them in the `target/site/apidocs` directory. 
+This will generate the API docs, and place them in the `target/site/apidocs` directory.
 
 To generate API docs for only the public APIs (i.e. ones that are not marked with `@hide` tags),
 you need to trigger the `devsite-apidocs` Maven profile. This profile uses Maven Javadoc plugin