chore: regenerate README (#253)

This PR was generated using Autosynth. 🌈


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

```
2020-10-16 00:16:46,565 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-iamcredentials/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working directory clean
2020-10-16 00:16:47,490 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/5838df64-ef18-4437-927c-238da7e3ccc3/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-iamcredentials.git",
-        "sha": "60d84d5b3b36b9f60e0848468320a60e87982ab7"
+        "sha": "da1659767e6411bf70e2a9927b8cd9207f871f11"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "27e0e916cbfdb3d5ff6639b686cc04f78a0b0386"
+        "sha": "7c5370937dd9ba9dcf9cd7d2af880a58b389b4f1"
       }
     }
   ]

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>12.0.0</version>
+      <version>12.1.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>