docs: add usage

README.md

@@ -22,6 +22,28 @@ For each netcup customer, create a `.json` configuration file inside `/etc/netcu
 
 There is an [example configuration](cfg/example.json).
 
+## Usage
+
+```
+usage: netcup-dns [-h] [--config-directory CFG_DIR]
+                  [--cache-directory CACHE_DIR]
+                  [--cache-validity-seconds CACHE_VALIDITY_SECONDS]
+
+Update DNS A/AAAA records with your current external IP address using the
+netcup DNS API.
+
+options:
+  -h, --help            show this help message and exit
+  --config-directory CFG_DIR
+                        Path to directory where `.json` config files reside.
+  --cache-directory CACHE_DIR
+                        Path to cache directory. Retrieved and updated DNS
+                        records are cached there.
+  --cache-validity-seconds CACHE_VALIDITY_SECONDS
+                        Value in seconds for how long cached DNS records are
+                        valid. Set to `0` to disable caching.
+```
+
 ## TODOs
 
 Alternative external IP detection:

src/netcup_dns/main.py

@@ -57,7 +57,8 @@ def main():
 
 
 def parse_args():
-    parser = argparse.ArgumentParser(description='Update DNS A/AAAA records with your current external IP address'
+    parser = argparse.ArgumentParser(prog='netcup-dns',
+                                     description='Update DNS A/AAAA records with your current external IP address'
                                                  ' using the netcup DNS API.')
     parser.add_argument('--config-directory',
                         help='Path to directory where `.json` config files reside.',
@@ -65,7 +66,7 @@ def parse_args():
                         default=Path('/etc/netcup-dns'),
                         type=Path)
     parser.add_argument('--cache-directory',
-                        help='Path to cache directory. Retrieved/Set DNS records are cached there.',
+                        help='Path to cache directory. Retrieved and updated DNS records are cached there.',
                         dest='cache_dir',
                         default=Path.home().joinpath('.netcup-dns/cache'),
                         type=Path)