chore: regenerate README (#535)

This PR was generated using Autosynth. 🌈


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

```
2021-02-25 00:17:52,707 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-pubsub/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-02-25 00:17:54,345 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/1019b399-a8a8-4315-8727-b8ad89c3d933/targets

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-pubsub.git",
-        "sha": "7622cdb24cd3428c94e90d584d0334eb27c51393"
+        "sha": "6f172ae74512bfbcb9d2c017ff6fff6ee0478755"
       }
     },
     {

README.md

@@ -12,6 +12,24 @@ Java idiomatic client for [Cloud Pub/Sub][product-docs].
 
 If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
 ```xml
+<dependencyManagement>
+  <dependencies>
+    <dependency>
+      <groupId>com.google.cloud</groupId>
+      <artifactId>libraries-bom</artifactId>
+      <version>17.0.0</version>
+      <type>pom</type>
+      <scope>import</scope>
+    </dependency>
+  </dependencies>
+</dependencyManagement>
+
+<dependencies>
+  <dependency>
+    <groupId>com.google.cloud</groupId>
+    <artifactId>google-cloud-pubsub</artifactId>
+  </dependency>
+
 </dependencies>
 
 ```
@@ -29,7 +47,7 @@ If you are using Maven without BOM, add this to your dependencies:
 
 If you are using Gradle 5.x or later, add this to your dependencies
 ```Groovy
-implementation platform('com.google.cloud:libraries-bom:17.0.0')
+implementation platform('com.google.cloud:libraries-bom:18.0.0')
 
 compile 'com.google.cloud:google-cloud-pubsub'
 ```