Add documentation on how to get bundlesize GitHub integration in generic CI

[thiagogoncales]

Do you want to request a feature or report a bug?
Feature

What is the current behavior?
No documentation on how to integrate bundlesize with some generic CI

If this is a feature request, what is motivation or use case for changing the behavior?
I had to dig through https://github.com/siddharthkp/ci-env code base in order to find the required variables, which btw are

• CI_REPO_OWNER
• CI_REPO_NAME
• CI_COMMIT_SHA
• BUNDLESIZE_GITHUB_TOKEN

With this, you can hook bundlesize up with Jenkins for instance, and get a bit closer to solving #22

[jakebolam]

THANK YOU, THIS NEEDS TO BE IN THE DOCS

[siddharthkp]

@jakebolam Do you want to add it?