Improve flagr documentation of clients, SDKs, and community contributions

[zhouzhuojie]

Currently the flagr clients are supported in 4 languages: https://checkr.github.io/flagr/#/

We'd like to include detailed documentation for

• how to generate production ready clients from the swagger yaml definitions
• how to include community contributed clients. (e.g. https://github.com/checkr/jsflagr/issues/8 js-browser-flagr)
• how to configure the client properly and match what you setup from the flagr server side
• potentially include integration tests from the official swagger generate clients (e.g. https://github.com/checkr/flagr/issues/241)