[Docs] Add Release Notes for 20.04
[Docs] Add Release Notes for 20.04

Resolves: LE-13892
Change-Id: Ie61355e777490e99b27f177572a01791d0889826

legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af