What are some alternatives?
When comparing ReadMe and GitBook, you can also consider the following products
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
Atlassian Bitbucket Server - Atlassian Bitbucket Server is a scalable collaborative Git solution.
Docusaurus - Easy to maintain open source documentation websites
Visual Studio Team Services - Visual Studio Team Services (VSTS) provides unlimited Git hosting, cloud build for CI, agile planning and continuous delivery.
GitHub Desktop - GitHub Desktop is a seamless way to contribute to projects on GitHub and GitHub Enterprise.