Merge pull request #3 from BoltApp/updateReadme

Updated Readme. Added description and examples of using PolicyTags

README.md

@@ -69,6 +69,105 @@ The message `foo.Baz` is ignored because it doesn't have option `gen_bq_schema.b
 `protoc --bq-schema_out=. --bq-schema_opt=single-message single_message.proto` will generate a file named `foo/single_message.schema`.
 The message `foo.Baz` is also ignored because it is not the first message in the file.
 
+
+### Support for PolicyTags
+`protoc-gen-bq-schema` now supports [policyTags](https://cloud.google.com/bigquery/docs/column-level-security-intro). 
+You can define a `Policy Tag` for a field in `.proto` file. 
+
+### Example with Policy Tags
+Suppose that you have the following `test_table.proto`
+```
+syntax = "proto3";
+package foo;
+import "bq_table.proto";
+import "bq_field.proto";
+
+message TestTable{
+    option (gen_bq_schema.bigquery_opts).table_name = "test_table";
+
+    int32 a = 1 [
+        (gen_bq_schema.bigquery) = {
+          require: true
+          policy_tags : "private"
+        }
+      ];
+    
+    string b = 2 [(gen_bq_schema.bigquery).policy_tags="public"];
+
+    message Nested {
+        int32 a = 1 [(gen_bq_schema.bigquery) = {
+            require: true
+            policy_tags : "private"
+            }
+        ];
+
+        string b = 2;
+    }
+
+    repeated Nested nested = 3 [(gen_bq_schema.bigquery).require = true];
+
+    message EmptyMessage {}
+
+    repeated EmptyMessage hasMessage = 4;
+}
+```
+`protoc --bq-schema_out=. test_table.proto` will generate a file named `foo/test_table.schema`.
+The field `hasMessage` is ignored because the message `EmptyMessage` is empty.
+
+It will generate the following `JSON` schema
+```
+[
+ {
+  "name": "a",
+  "type": "INTEGER",
+  "mode": "REQUIRED",
+  "policyTags": {
+   "names": [
+    "private"
+   ]
+  }
+ },
+ {
+  "name": "b",
+  "type": "STRING",
+  "mode": "NULLABLE",
+  "policyTags": {
+   "names": [
+    "public
+   ]
+  }
+ },
+ {
+  "name": "nested",
+  "type": "RECORD",
+  "mode": "REQUIRED",
+  "fields": [
+   {
+    "name": "a",
+    "type": "INTEGER",
+    "mode": "REQUIRED",
+    "policyTags": {
+     "names": [
+      "private"
+     ]
+    }
+   },
+   {
+    "name": "b",
+    "type": "STRING",
+    "mode": "NULLABLE"
+   }
+  ]
+ }
+]
+```
+
+The policy tag name provided in `test_table.proto` file is taken as it is. According to [Google Docs](https://cloud.google.com/bigquery/docs/column-level-security-intro), 
+the policy tag string should be of the following format
+
+`projects/project-id/locations/location/taxonomies/taxonomy-id/policyTags/policytag-id`
+
+
 ## License
 
 protoc-gen-bq-schema is licensed under the Apache License version 2.0.