Update generate command usage in README

Author: mattt

README.md

@@ -67,7 +67,7 @@ $ make install
 
     OVERVIEW: Generates Swift documentation
 
-    USAGE: swift-doc generate [<inputs> ...] --module-name <module-name> [--output <output>] [--format <format>]
+    USAGE: swift-doc generate [<inputs> ...] --module-name <module-name> [--output <output>] [--format <format>] [--base-url <base-url>]
 
     ARGUMENTS:
       <inputs>                One or more paths to Swift files 
@@ -78,6 +78,8 @@ $ make install
       -o, --output <output>   The path for generated output (default:
                               .build/documentation)
       -f, --format <format>   The output format (default: commonmark)
+      --base-url <base-url>   The base URL used for all relative URLs in generated
+                              documents. (default: /)
       -h, --help              Show help information.
 
 The `generate` subcommand