Specify Secrets.toml format in Docs. (#114)

* Specify Secrets.toml format in Docs.

* Update shuttle-secrets.mdx

resources/shuttle-secrets.mdx

@@ -8,6 +8,13 @@ This plugin manages secrets on [Shuttle](https://www.shuttle.rs).
 Add `shuttle-secrets` to the dependencies for your service, and add a `Secrets.toml` to the root of your project
 with the secrets you'd like to store. Make sure to add `Secrets.toml` to a `.gitignore` to omit your secrets from version control.
 
+The format of the Secrets.toml file is a key-value mapping with string values.
+
+```toml
+MY_API_KEY = 'the contents of my API key'
+MY_OTHER_SECRET = 'some other secret'
+```
+
 Next, pass `#[shuttle_secrets::Secrets] secret_store: SecretStore` as an argument to your `shuttle_runtime::main` function.
 `SecretStore::get` can now be called to retrieve your API keys and other secrets at runtime.
 
@@ -19,6 +26,7 @@ If you don't have a `Secrets.dev.toml` file, `Secrets.toml` will be used locally
 both secret files with some of the same secrets for both local runs and deployments, you have to duplicate the secret across both 
 files.
 
+
 ## Example
 This snippet shows a Shuttle rocket main function that uses the `shuttle_secrets::Secrets` attribute to gain access to a `SecretStore`.