Add[#204]: an example to create basic RDS database

[vivekvr1]

Closes: #204

Be noted that 204 is the issue ID

What is the purpose of the change

• This pull request adds an example to create a basic RDS database in the getting-started/rds/create-simple-rds directory. This will help users to learn how to create and manage RDS databases using Terraform.

Add a description of the overall background and high level changes that this PR introduces

• Added a new directory called getting-started/rds/create-simple-rds.
• Added a new file called main.tf to the getting-started/rds/create-simple-rds directory.

[tungbq]

LGTM. Thanks @vivekvr1

[tungbq]

It seems that the code you provided did not work. @vivekvr1
I've created another fix to make it's working.

• Result
  

https://github.com/tungbq/aws-lab-with-terraform/pull/273/files

It should be better if we could do a trial run on the code of the pull request before requesting a review