[README] Add optional directory parameter for the init command (#1409)

With the current description of the command, I was expecting to get a directory named with the project name, but the files were created in the current directory.
I Think a more precise description would help first-time users.

Author: maximeborges

README.md

@@ -91,10 +91,11 @@ all its functionality as a Rust crate for integration in other projects.
 Here are the main commands you will want to run. For a more exhaustive
 explanation, check out the [User Guide].
 
-- `mdbook init`
+- `mdbook init <directory>`
 
     The init command will create a directory with the minimal boilerplate to
-    start with.
+    start with. If the `<directory>` parameter is omitted, the current 
+    directory will be used.
 
     ```
     book-test/