Readme update. Virtual Machine use remark

Author: PrzemekWirkus

README.md

@@ -531,4 +531,5 @@ OK
 ```
 
 # Known issues
+* Users reported issues while using ```mbed-ls``` on VM (Virtual Machines).
 * [mbedls fails to list devices on OS X El Capitan](https://github.com/ARMmbed/mbed-ls/issues/38).