Updated readme

Readme.md

@@ -15,13 +15,17 @@ npm install base62
 
 ## Usage
 
+### Default Character Set Example
+
 ```javascript
 Base62 = require('base62')
 Base62.encode(999)  // 'g7'
 Base62.decode('g7') // 999
 ```
 
-The default character set is `0-9a-zA-Z`. This can updated to a custom character set. Naturally, it must be 62 characters long.
+### Custom Character Set Example
+
+The default character set is `0-9a-zA-Z`. This can be updated to a custom character set. Naturally, it must be 62 characters long.
 
 Instead of the character set `0-9a-zA-Z` you want to use `0-9A-Za-z`, call the `setCharacterSet()` method on the Base62 object passing in the string `"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"`. Note that all characters must be unique.