docs: add docs on validation, fix TOC (#405)

* docs: add docs on validation, fix TOC

* chore: move validation subsection to bottom of openapi section

* docs: add YAML callout

README.md

@@ -11,7 +11,7 @@
    * [Authentication](#authentication)
 * [Usage](#usage)
    * [Common options](#common-rdme-options)
-   * [Swagger / OpenAPI](#swagger-/-openapi)
+   * [OpenAPI / Swagger](#openapi--swagger)
    * [Docs](#docs)
    * [Versions](#versions)
    * [Opening a Project](#open)
@@ -42,6 +42,8 @@ If you wish to get more information about any command within `rdme`, you can exe
 ### OpenAPI / Swagger
 ReadMe supports [OpenAPI 3.0](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.3.md), [OpenAPI 3.1](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.1.0.md), and [Swagger 2.x](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/2.0.md).
 
+The following examples use JSON files, but we support API Definitions that are written in either JSON or YAML.
+
 > ℹ️ Note that the `rdme openapi` command supports both OpenAPI and Swagger API definitions. The `rdme swagger` command is an alias for `rdme openapi` and is deprecated.
 
 #### Uploading a New API Definition to ReadMe
@@ -76,6 +78,13 @@ If you run `rdme` within a directory that contains your OpenAPI or Swagger defin
 rdme openapi
 ```
 
+#### Validating an API Definition
+You can also perform a local validation of your API definition without uploading it to ReadMe, which can be useful when constructing or editing your API definition.
+
+```sh
+rdme validate [path-to-file.json]
+```
+
 ### Docs
 #### Syncing a Folder of Markdown Docs to ReadMe
 ```sh