add help for each argument

src/cli.rs

@@ -35,15 +35,30 @@ under certain conditions;
 "
 )]
 pub struct Args {
-    #[clap(short = 'f', long, default_value = ".env")]
+    #[clap(
+        short = 'f',
+        long,
+        default_value = ".env",
+        help = "Change source file with environment variables"
+    )]
     pub file: PathBuf,
 
-    #[clap(short = 'o', long)]
+    #[clap(
+        short = 'o',
+        long,
+        help = "Change output file with modified environment variables. It uses `file` argument by default if the output is not specified"
+    )]
     pub output: Option<PathBuf>,
 
-    #[clap(long)]
+    #[clap(
+        long,
+        help = "Writes modified environment variables to stdout instead of a file. `Output` argument will be ignored."
+    )]
     pub dry_run: bool,
 
-    #[clap(value_parser)]
+    #[clap(
+        value_parser,
+        help = "Environment varible sections that will be enabled"
+    )]
     pub sections: Vec<String>,
 }