Add link to README themes documentation in --help text

src/cli.rs

@@ -269,12 +269,15 @@ pub struct Opt {
     #[structopt(long = "show-syntax-themes")]
     pub show_syntax_themes: bool,
 
-    /// Show available delta themes, each with an example of highlighted diff output. A delta theme
-    /// is a delta named feature (see --features) that sets either `light` or `dark`. If diff
-    /// output is supplied on standard input then this will be used for the demo. For example: `git
-    /// show | delta --show-themes`. By default shows dark or light themes only, according to
-    /// whether delta is in dark or light mode (as set by the user or inferred from BAT_THEME). To
-    /// control the themes shown, use --dark or --light, or both, on the command line together with
+    /// Show available delta themes, each with an example of highlighted diff
+    /// output. A delta theme is a delta named feature (see --features) that
+    /// sets either `light` or `dark`. See
+    /// https://github.com/dandavison/delta#custom-color-themes. If diff output
+    /// is supplied on standard input then this will be used for the demo. For
+    /// example: `git show | delta --show-themes`. By default shows dark or
+    /// light themes only, according to whether delta is in dark or light mode
+    /// (as set by the user or inferred from BAT_THEME). To control the themes
+    /// shown, use --dark or --light, or both, on the command line together with
     /// this option.
     #[structopt(long = "show-themes")]
     pub show_themes: bool,