Add short section for resources in rdb namespace

[remyleone]

Description:

How to reproduce:

Command attempted

scw rdb

Expected Behavior

An explanation of what each resource is about

Actual Behavior

$ go run ./cmd/scw/main.go rdb         
Database RDB API

USAGE:
  scw rdb <command>

AVAILABLE COMMANDS:
  backup      
  engine      
  instance    
  acl         
  settings    
  privilege   
  user        
  database    
  node-type   
  log         
  certificate 

More info

Version

version 2.0.0-beta.4+dev
build-date unknown
go-version go1.14.4
git-branch unknown
git-commit unknown
go-arch amd64
go-os darwin