Add required paragraph field to library.properties

Missing paragraph field causes the Arduino IDE to display a warning whenever the library is compiled or Library/Boards Manager is opened/closed:

Invalid library found in E:\arduino\libraries\serde: Missing 'paragraph' from library

This also results in the library's examples not being shown in the File > Examples menu. With older versions of the Arduino IDE, it caused compilation of the library to fail.

library.properties

@@ -3,6 +3,7 @@ version=0.1.0
 author=Francois Best
 maintainer=Francois Best <[email protected]>
 sentence=Exchange structured data between Arduino boards
+paragraph=
 category=Communication
 url=https://github.com/FortySevenEffects/serde
 architectures=*