Add update instructions in README

[trivikr]

Is your feature request related to a problem? Please describe.

• There are no update instructions for projects who're already using github-codebuild-logs
• Installation instructions are in README https://github.com/jlhood/github-codebuild-logs#installation-instructions

Describe the solution you'd like
Add update instructions in README

Describe alternatives you've considered
Deleting application stack, and redeploying a new version

[trivikr]

Example difference in comments when the old application stack was deleted, and new one was deployed:

• Old: chore(deps-dev): bump yarn from 1.22.0 to 1.22.4 aws/aws-sdk-js-v3#989 (comment)
• New: chore(deps-dev): bump yarn from 1.22.0 to 1.22.4 aws/aws-sdk-js-v3#989 (comment)

Deleting old stack deletes links to the build logs created using older application which might be a concern for some projects. The instructions can be provided for updating the application instead.

[trivikr]

Confirmed that deletion of old Cloudformation stack is not required.
One can just overwrite previous Cloudformation stack, and old resources will be preserved.

It would be good to have update instructions in README though.