Add document for external table (Snowflake-Labs#415)

document for external table resource is missing in the main branch.

<!-- summary of changes -->

## Test Plan
<!-- detail ways in which this PR has been tested or needs to be tested -->
* [x] acceptance tests
<!-- add more below if you think they are relevant -->
* [ ] …

## References
<!-- issues documentation links, etc  -->

*

docs/resources/external_table.md

@@ -9,7 +9,26 @@ description: |-
 
 
 
-
+## Example Usage
+
+```terraform
+resource snowflake_external_table external_table {
+  database = "db"
+  schema   = "schema"
+  name     = "external_table"
+  comment  = "External table"
+
+  column {
+    name = "id"
+    type = "int"
+  }
+
+  column {
+    name = "data"
+    type = "text"
+  }
+}
+```
 
 ## Schema
 
@@ -45,4 +64,11 @@ Required:
 - **name** (String, Required) Column name
 - **type** (String, Required) Column type, e.g. VARIANT
 
+## Import
+
+Import is supported using the following syntax:
 
+```shell
+# format is database name | schema name | external table name
+terraform import snowflake_external_table.example 'dbName|schemaName|externalTableName'
+```

examples/resources/snowflake_external_table/import.sh

@@ -0,0 +1,2 @@
+# format is database name | schema name | external table name
+terraform import snowflake_external_table.example 'dbName|schemaName|externalTableName'

examples/resources/snowflake_external_table/resource.tf

@@ -0,0 +1,16 @@
+resource snowflake_external_table external_table {
+  database = "db"
+  schema   = "schema"
+  name     = "external_table"
+  comment  = "External table"
+
+  column {
+    name = "id"
+    type = "int"
+  }
+
+  column {
+    name = "data"
+    type = "text"
+  }
+}