Update README.md

[dvhoang-amazon]

what

• Describe high-level what changed as a result of these commits (i.e. in plain-english, what do these changes mean?)
• Use bullet points to be concise and to the point.

why

• Provide the justifications for the changes (e.g. business case).
• Describe why these changes were made (e.g. why do these commits fix the problem?)
• Use bullet points to be concise and to the point.

references

• Link to any supporting github issues or helpful documentation to add some context (e.g. stackoverflow).
• Use closes #123, if this PR closes a GitHub issue #123

[nitrocode]

@dvhoang-amazon You'll need to update the variable descriptions in the variables.tf file for those changes to be put into the README.md