chore: regenerate README (googleapis#337)

This PR was generated using Autosynth. 🌈


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

```
2021-01-31 23:34:10,379 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner-jdbc/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-01-31 23:34:11,331 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/c457d563-a78f-4df4-9e52-78d49fa9db85/targets

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

Author: yoshi-automation

java-spanner-jdbc/.github/readme/synth.metadata/synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner-jdbc.git",
-        "sha": "5115ed8f45b4fe1c0424d25190cdfc8681ef388f"
+        "sha": "8b4e50d10a9121334be3d8b5ed0d8fc9ff63c182"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "a3e990f3545dc8ccd384a75d20ce9cb185ca6a28"
+        "sha": "d1bb9173100f62c0cfc8f3138b62241e7f47ca6a"
       }
     }
   ]

java-spanner-jdbc/README.md

@@ -84,6 +84,16 @@ try (Connection connection =
 }
 ```
 
+### Creating a Shaded Jar
+
+A jar with all dependencies included is automatically generated when you execute `mvn package`.
+The dependencies in this jar are not shaded. To create a jar with shaded dependencies you must
+activate the `shade` profile like this:
+
+ ```
+ mvn package -Pshade
+ ```
+