chore: regenerate README (#151)

This PR was generated using Autosynth. 🌈


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

```
2021-06-14 16:23:02,757 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-pubsublite-kafka/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-06-14 16:23:04,078 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/9ae6a297-147a-4d31-ba39-4888432f7790/targets

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-pubsublite-kafka.git",
-        "sha": "31b7ab9ccbe8407a06a1a91e748e1e45b2dbd229"
+        "sha": "b8b95394a64063cc6488ec73a807e260cbe3a727"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "c3e41da0fa256ad7f6b4bc76b9d069dedecdfef4"
+        "sha": "41ccd8cd13ec31f4fb839cf8182aea3c7156e19d"
       }
     }
   ]

README.md

@@ -20,18 +20,18 @@ If you are using Maven, add this to your pom.xml file:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>pubsublite-kafka</artifactId>
-  <version>0.2.3</version>
+  <version>0.4.0</version>
 </dependency>
 ```
 
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:pubsublite-kafka:0.2.3'
+compile 'com.google.cloud:pubsublite-kafka:0.4.0'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "pubsublite-kafka" % "0.2.3"
+libraryDependencies += "com.google.cloud" % "pubsublite-kafka" % "0.4.0"
 ```
 
 ## Authentication