chore: regenerate README (googleapis#542)

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<details><summary>Log from Synthtool</summary>

```
2021-07-27 20:52:25,206 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner-jdbc/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-07-27 20:52:26,466 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

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.github/readme/synth.metadata/synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner-jdbc.git",
-        "sha": "a4bd82c8e4ce8b7179b943ac06b049598276f1b4"
+        "sha": "68125c4d896a24f186d90648d3be64b5ad15aa37"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "959d4f81f2d1397297ebda90212df92aa0ef3a6a"
+        "sha": "3d32990b6f4b57be77455e3ca89e6e21cd0c06b3"
       }
     }
   ]

README.md

@@ -8,11 +8,13 @@ Java idiomatic client for [Google Cloud Spanner JDBC][product-docs].
 - [Product Documentation][product-docs]
 - [Client Library Documentation][javadocs]
 
+
 ## Quickstart
 
 
 If you are using Maven, add this to your pom.xml file:
 
+
 ```xml
 <dependency>
   <groupId>com.google.cloud</groupId>
@@ -22,11 +24,13 @@ If you are using Maven, add this to your pom.xml file:
 ```
 
 If you are using Gradle without BOM, add this to your dependencies
+
 ```Groovy
 compile 'com.google.cloud:google-cloud-spanner-jdbc:2.2.6'
 ```
 
 If you are using SBT, add this to your dependencies
+
 ```Scala
 libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "2.2.6"
 ```
@@ -35,6 +39,10 @@ libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "2.2.6
 
 See the [Authentication][authentication] section in the base directory's README.
 
+## Authorization
+
+The client application making API calls must be granted [authorization scopes][auth-scopes] required for the desired Google Cloud Spanner JDBC APIs, and the authenticated principal must have the [IAM role(s)][predefined-iam-roles] required to access GCP resources using the Google Cloud Spanner JDBC API calls.
+
 ## Getting Started
 
 ### Prerequisites
@@ -178,6 +186,7 @@ and on [google-cloud-java][g-c-j].
 This library follows [Semantic Versioning](http://semver.org/).
 
 
+
 ## Contributing
 
 
@@ -189,6 +198,7 @@ Please note that this project is released with a Contributor Code of Conduct. By
 this project you agree to abide by its terms. See [Code of Conduct][code-of-conduct] for more
 information.
 
+
 ## License
 
 Apache 2.0 - See [LICENSE][license] for more information.
@@ -220,6 +230,9 @@ Java is a registered trademark of Oracle and/or its affiliates.
 [maven-version-image]: https://img.shields.io/maven-central/v/com.google.cloud/google-cloud-spanner-jdbc.svg
 [maven-version-link]: https://search.maven.org/search?q=g:com.google.cloud%20AND%20a:google-cloud-spanner-jdbc&core=gav
 [authentication]: https://github.com/googleapis/google-cloud-java#authentication
+[auth-scopes]: https://developers.google.com/identity/protocols/oauth2/scopes
+[predefined-iam-roles]: https://cloud.google.com/iam/docs/understanding-roles#predefined_roles
+[iam-policy]: https://cloud.google.com/iam/docs/overview#cloud-iam-policy
 [developer-console]: https://console.developers.google.com/
 [create-project]: https://cloud.google.com/resource-manager/docs/creating-managing-projects
 [cloud-sdk]: https://cloud.google.com/sdk/