docs: explain default generation config in README.md (#3388)

Author: JoeWang1127

hermetic_build/README.md

@@ -18,20 +18,36 @@ for additional instructions.
 
 ## Prerequisites
 
-In order to generate a version for each library, a versions.txt has to exist
+### Generation configuration
+
+A generation configuration file is required to generate GAPIC Client Libraries.
+
+Please refer to [Generation configuration yaml](#generation-configuration-yaml--generation-config-path---optional)
+for more information.
+
+### versions.txt
+
+In order to generate a version for each library, a `versions.txt` has to exist
 in `repository-path`.
 Please refer to [Repository path](#repository-path--repositorypath---optional)
 for more information.
 
 ## Parameters to generate a repository using the docker image
 
-### Generation configuration yaml (`generation-config-path`)
+### Generation configuration yaml (`generation-config-path`), optional
 
 An absolute or relative path to a configuration file containing parameters to
 generate the repository.
 Please refer to [Configuration to generate a repository](#configuration-to-generate-a-repository)
 for more information.
 
+The default value is `$(pwd)/generation_config.yaml`, i.e., `generation_config.yaml`
+in the current working directory.
+
+This will raise `FileNotFoundError` if the specified generation config does not
+exist or, in case `generation-config-path` is not specified, the default
+generation config does not exist.
+
 ### Repository path (`repository-path`), optional
 
 The path to where the generated repository goes.
@@ -41,8 +57,8 @@ For example, `cd google-cloud-java && python /path/to/entry_point.py ...` withou
 specifying the `--repository_path` option will modify the `google-cloud-java`
 repository the user `cd`'d into.
 
-Note that versions.txt has to exist in `repository_path` in order to generate
-right version for each library.
+Note that `versions.txt` has to exist in `repository_path` in order to generate
+the right version for each library.
 Please refer [here](go/java-client-releasing#versionstxt-manifest) for more info
 of versions.txt.
 

hermetic_build/library_generation/cli/entry_point.py

@@ -78,28 +78,18 @@ def generate(
     api_definitions_path: str,
 ):
     """
-    Compare baseline generation config and current generation config and
-    generate changed libraries based on current generation config.
+    Generate libraries based on a generation config.
 
-    If baseline generation config is not specified but current generation
-    config is specified, generate all libraries if `library_names` is not
-    specified, based on current generation config.
-
-    If current generation config is not specified but baseline generation
-    config is specified, raise FileNotFoundError because current generation
-    config should be the source of truth of library generation.
-
-    If both baseline generation config and current generation config are not
-    specified, generate all libraries based on the default generation config,
-    which is generation_config.yaml in the current working directory.
+    If the `generation-config-path` is not specified the default generation
+    config, `$(pwd)/generation_config.yaml`, will be used.
 
     If `library_names` is specified, only libraries whose name can be found in
-    the current generation config or default generation config, if current
-    generation config is not specified, will be generated. Changed libraries
-    will be ignored even if baseline and current generation config are
-    specified.
+    the generation config will be generated; otherwise all libraries in the
+    generation config will be generated.
 
-    Raise FileNotFoundError if the default config does not exist.
+    :raise FileNotFoundError if the specified generation config does not exist
+    or, in case `generation-config-path` is not specified, the default
+    generation config does not exist.
     """
     __generate_repo_impl(
         generation_config_path=generation_config_path,