docs: mode feature (#16)

Reviewed-on: #16
Co-authored-by: Timo Behrendt <t.behrendt@t00n.de>
Co-committed-by: Timo Behrendt <t.behrendt@t00n.de>

README.md

@@ -1,10 +1,12 @@
 # realDynDNS
+
 RealDynDNS aims to be a replacement to "classical" dynDNS solutions that offer a subdomain. Instead realDynDns actually changes your DNS entries.
 
 This service requires your DNS provider to expose an API that allows your DNS entries to be changed.
 A service that provides your current external IP is also required.
 
 ## Configuration
+
 The configuration is done via a YAML file called `config.yaml`. The following example shows the configuration for a domain with two subdomains.
 
 Configuration of the IP provider and the DNS provider is mandatory.
@@ -35,40 +37,51 @@ domains:
       - "@"
       - www
 check_interval: 0 0 0/6 * * * *
+mode: Scheduled
 ```
 
 The config file is expected to be in the same directory as the binary and called `config.yaml`. For the OCR image, the root directory is `/app`.
 
 ## DNS Providers
+
 The DNS provider abstracts the API of your DNS provider. Currently the following providers are supported:
 
 ### IONOS
+
 IONOS requires two configuration parameters. You can get your API key [here](https://developer.hosting.ionos.com/docs/getstarted).
+
 ```yaml
 api_key: <your-api-key>
 base_url: https://api.hosting.ionos.com/dns
 ```
 
 ## External IP Providers
+
 The external IP provider is used to get your current external IP. Currently the following providers are supported:
 
 ### Plain
+
 Any provider that returns your IP as plain text can be used. The following configuration is required:
+
 ```yaml
 url: <your-providers-URL>
 ```
 
 Examples for providers are:
+
 - https://ifconfig.me
 - https://api.ipify.org
 
 ## Notification Providers
+
 The notification provider is used to send notifications whena IP address changes and a DNS record is updated.
 
 ### Console
+
 The console notification provider is used to print the notification to the console. This is the default notification provider.
 
 ### Gotify
+
 The Gotify notification provider is used to send notifications to a Gotify server.
 
 ```yaml
@@ -77,4 +90,20 @@ token: <your-token>
 priority: 0
 ```
 
-The priority must be between 0 and 4.
+The priority must be between 0 and 4.
+
+## Mode Selection
+
+Two modes are available:
+
+### RunOnce
+
+The RunOnce mode is used to run the application once and exit. This is useful when providing your own external scheduler, like cron.
+
+Set the `mode` to `RunOnce`.
+
+### Scheduled
+
+The Scheduled mode is used to run the application in a scheduled interval.
+
+Set the `mode` to `Scheduled` and provide a cron expression for the `check_interval`.