chore: update CONTRIBUTING.md to include code formatting (#99)

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Source-Link: googleapis/synthtool@5b48b07

Author: yoshi-automation

CONTRIBUTING.md

@@ -99,7 +99,16 @@ mvn -Penable-samples clean verify
     ```
 
 2. [Activate](#profile-activation) the profile.
-3. Define your samples in a normal Maven project in the `samples/` directory
+3. Define your samples in a normal Maven project in the `samples/` directory.
+
+### Code Formatting
+
+Code in this repo is formatted with
+[google-java-format](https://github.com/google/google-java-format).
+To run formatting on your project, you can run:
+```
+mvn com.coveo:fmt-maven-plugin:format
+```
 
 ### Profile Activation
 

synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-billing.git",
-        "sha": "50afb0e301f97194bf58c78b0133606cf140784f"
+        "sha": "084f64f073bfa583f4fc5d0218ee32dd9ef832c2"
       }
     },
     {
@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "98c50772ec23295c64cf0d2ddf199ea52961fd4c"
+        "sha": "5b48b0716a36ca069db3038da7e205c87a22ed19"
       }
     }
   ],