chore: regenerate README (#494)

This PR was generated using Autosynth. 🌈


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

```
2021-01-07 21:01:58,564 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-firestore/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-01-07 21:01:59,456 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/dc2fe467-4dff-46eb-9fe1-202347a288d1/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": "e4aa8ce96693f52340209c5d445ae13bef26bc1f"
+        "sha": "6e15f54e327398460c43e5959e2b0e13e33a0a8e"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "996775eca5fd934edac3c2ae34b80ff0395b1717"
+        "sha": "41a4e56982620d3edcf110d76f4fcdfdec471ac8"
       }
     }
   ]

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>16.2.0</version>
+      <version>16.2.1</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>
@@ -43,10 +43,17 @@ If you are using Maven without BOM, add this to your dependencies:
 
 ```
 
-If you are using Gradle, 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:16.2.1')
+
+compile 'com.google.cloud:google-cloud-firestore'
+```
+If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
 compile 'com.google.cloud:google-cloud-firestore:2.1.0'
 ```
+
 If you are using SBT, add this to your dependencies
 ```Scala
 libraryDependencies += "com.google.cloud" % "google-cloud-firestore" % "2.1.0"