add explanatory example for need of Unit

docs/unit.md

@@ -8,6 +8,16 @@ description: Utility to easily convert between bitcoin units.
 
 Unit is a utility for handling and converting bitcoin units. We strongly recommend to always use satoshis to represent amount inside your application and only convert them to other units in the front-end.
 
+To understand the need of using the `Unit` class when dealing with unit conversions, see this example:
+```
+> 81.99 * 100000 // wrong
+8198999.999999999
+> var bitcore = require('bitcore');
+> var Unit = bitcore.Unit;
+> Unit.fromMilis(81.99).toSatoshis() // correct
+8199000
+```
+
 ## Supported units
 
 The supported units are BTC, mBTC, bits (micro BTCs, uBTC) and satoshis. The codes for each unit can be found as members of the Unit class.