maven_install.json needs instructions for regenerating the file

somewhere obvious in this file it should say that I should run `bazel run @unpinned_maven//:pin` if I need to make a change. I had to ask a teammate for help on this one.

```
{
"dependency_tree": {
"__AUTOGENERATED_FILE_DO_NOT_MODIFY_THIS_FILE_MANUALLY": "THERE_IS_NO_DATA_ONLY_ZUUL",
"__INPUT_ARTIFACTS_HASH": -1650978854,
"__RESOLVED_ARTIFACTS_HASH": -718494898,
"conflict_resolution": {
"com.google.errorprone:error_prone_annotations:2.3.2": "com.google.errorprone:error_prone_annotations:2.5.1",
"junit:junit:4.12": "junit:junit:4.13.1"
},
```

Start with maven_install.json and inspect the existing autogenerated-file notice and surrounding metadata. Add an obvious note that changes should be regenerated with `bazel run @unpinned_maven//:pin`; done means a contributor can find the command in the file without asking for help.

Written by the indexing model from the issue text.