adding attribute family for bigtable_table (hashicorp#2228)

Fix hashicorp#1576

docs/r/bigtable_table.html.markdown

@@ -41,9 +41,17 @@ The following arguments are supported:
 
 * `split_keys` - (Optional) A list of predefined keys to split the table on.
 
+* `column_family` - (Optional) A group of columns within a table which share a common configuration. This can be specified multiple times. Structure is documented below.
+
 * `project` - (Optional) The ID of the project in which the resource belongs. If it
     is not provided, the provider project is used.
 
+-----
+
+`column_family` supports the following arguments:
+
+* `family` - (Optional) Creates a new column family in a table.
+
 ## Attributes Reference
 
 Only the arguments listed above are exposed as attributes.