Add documentation and example for description config

Author: jessestricker

book-example/book.json

@@ -1,4 +1,5 @@
 {
     "title": "mdBook Documentation",
+    "description": "Create book from markdown files. Like Gitbook but implemented in Rust",
     "author": "Mathieu David"
 }

book-example/src/format/config.md

@@ -8,6 +8,7 @@ Here is an example of what a ***book.json*** file might look like:
 {
     "title": "Example book",
     "author": "Name",
+    "description": "The example book covers examples.",
     "dest": "output/my-book"
 }
 ```
@@ -16,6 +17,7 @@ Here is an example of what a ***book.json*** file might look like:
 
 - **title:** title of the book
 - **author:** author of the book
+- **description:** description, which is added as meta in the html head of each page.
 - **dest:** path to the directory where you want your book to be rendered. If a relative path is given it will be relative to the parent directory of the source directory
 
 ***note:*** *the supported configurable parameters are scarce at the moment, but more will be added in the future*