chore: regenerate README (#445)

This PR was generated using Autosynth. 🌈


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

```
2020-10-30 23:36:18,474 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-firestore/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working directory clean
2020-10-30 23:36:19,418 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/ee307e65-e40b-49be-8483-2547b68d9b63/targets

- [ ] To automatically regenerate this PR, check this box.

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-firestore.git",
-        "sha": "a86ba5965280b260736daf4201cdfe31a7ba71e3"
+        "sha": "2399f48d1961a673fc6460820bfe0103d92e2cfe"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "fd3584b01cedd6c9f79d08be4e2365085f955aa5"
+        "sha": "b19b401571e77192f8dd38eab5fb2300a0de9324"
       }
     }
   ]

README.md

@@ -17,7 +17,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>13.3.0</version>
+      <version>13.4.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>