docs: document inline file contents

docs/configuration-v1_0.md

@@ -55,7 +55,8 @@ The Fedora CoreOS configuration is a YAML document conforming to the following s
     * **_overwrite_** (boolean): whether to delete preexisting nodes at the path. `source` must be specified if `overwrite` is true. Defaults to false.
     * **_contents_** (object): options related to the contents of the file.
       * **_compression_** (string): the type of compression used on the contents (null or gzip). Compression cannot be used with S3.
-      * **_source_** (string): the URL of the file contents. Supported schemes are `http`, `https`, `tftp`, `s3`, and [`data`][rfc2397]. When using `http`, it is advisable to use the verification option to ensure the contents haven't been modified. If source is omitted and a regular file already exists at the path, Ignition will do nothing. If source is omitted and no file exists, an empty file will be created.
+      * **_source_** (string): the URL of the file contents. Supported schemes are `http`, `https`, `tftp`, `s3`, and [`data`][rfc2397]. When using `http`, it is advisable to use the verification option to ensure the contents haven't been modified. If source is omitted and a regular file already exists at the path, Ignition will do nothing. If source is omitted and no file exists, an empty file will be created. Mutually exclusive with `inline`.
+      * **_inline_** (string): the contents of the file. Mutually exclusive with `source`.
       * **_verification_** (object): options related to the verification of the file contents.
         * **_hash_** (string): the hash of the config, in the form `<type>-<value>` where type is `sha512`.
     * **_append_** (list of objects): list of contents to be appended to the file. Follows the same stucture as `contents`