chore: add simple readme for generator (#2633)

Fixes: #2626

google-api-go-generator/README.md

@@ -0,0 +1,61 @@
+# google-api-go-generator
+
+This package is the discovery client generator for Go. It is used to generate
+all of the client libraries in this repository
+
+## Flags
+
+- `api` (`string`): The API ID to generate, like 'tasks:v1'. A value of `*` means
+  all. (default "*")
+- `api_json_file` (`string`): If non-empty, the path to a local file on disk
+  containing the API to generate. Exclusive with setting --api.
+- `api_pkg_base` (`string`):Go package prefix to use for all generated APIs.
+  (default "google.golang.org/api")
+- `base_url` (`string`): (optional) Override the default service API URL. If empty,
+  the service's root URL will be used.
+- `build` (`bool`): Compile generated packages.
+- `cache` (`bool`): Use cache of discovered Google API discovery documents.
+  (default true)
+- `copyright_year` (`string`): Year for copyright. (default "2024")
+- `discoveryurl` (`string`): URL to root discovery document (default
+  "www.googleapis.com/discovery/v1/apis")
+- `gendir` (`string`): Directory to use to write out generated Go files
+- `gensupport_pkg` (`string`): Go package path of the 'api/internal/gensupport'
+  support package. (default "google.golang.org/api/internal/gensupport")
+- `googleapi_pkg` (`string`): Go package path of the 'api/googleapi' support
+  package. (default "google.golang.org/api/googleapi")
+- `header_path` (`string`): If non-empty, prepend the contents of this file to
+  generated services.
+- `htransport_pkg` (`string`): Go package path of the 'api/transport/http' support
+  package. (default "google.golang.org/api/transport/http")
+- `install` (`bool`): Install generated packages.
+- `internal_pkg` (`string`): Go package path of the 'internal' support package.
+  (default "google.golang.org/api/internal")
+- `internaloption_pkg` (`string`): Go package path of the
+  'api/option/internaloption' support package. (default
+  "google.golang.org/api/option/internaloption")
+- `option_pkg` (`string`): Go package path of the 'api/option' support package.
+  (default "google.golang.org/api/option")
+- `output` (`string`): (optional) Path to source output file. If not specified,
+  the API name and version are used to construct an output path (e.g. tasks/v1).
+- `publiconly` (`bool`): Only build public, released APIs. Only applicable for
+  Google employees. (default true)
+
+## Automated usage
+
+This generator runs daily via the automated bash script:
+[discogen.sh](../internal/kokoro/discogen.sh).
+
+## Example local usage
+
+### Run same command the automation uses
+
+`make all`
+
+### Generate a client from a local discovery document
+
+`go build -o google-api-go-generator && ./google-api-go-generator -cache=true -install -api_json_file=/path/to/file`
+
+### Refresh an existing client
+
+`go build -o google-api-go-generator && ./google-api-go-generator -cache=false -install -api=sevicename:vsomething -gendir=..`