Adding changelog with github_changelog_generator gem

[gutoarraes]

This PR works on solving issue 11, which is the request for a changelog.

I created the changelog with the github_changelog_generator gem.

Future changes to the changelog can be done with github_changelog_generator -u serpapi -p turbo_tests -t <your-github-token> -b changelog.md. The token may not be necessary if there are less than 50 tokens required for the update.

I'm thinking of editing the README to include instructions on updating the changelog when a new version is rolled out, please let me know if this is a good idea @ilyazub.