Add a "repository SLA" to README.
Add a "repository SLA" to README.

This commit adds a repository service-level agreement that outlines the
maintainers' commitment to community contributors for both pull requests and
issues.

johnbeisner/cli,johnbeisner/cli,livarcocc/cli-1,dasMulli/cli,livarcocc/cli-1,livarcocc/cli-1,dasMulli/cli,johnbeisner/cli,dasMulli/cli