Please add feign's repository link to the project reference

[DevDengChao]

Is your feature request related to a problem? Please describe.

https://docs.spring.io/spring-cloud-openfeign/docs/current/reference/html/#spring-cloud-feign-overriding-defaults says:

The OkHttpClient and ApacheHttpClient and ApacheHC5 feign clients can be used by setting feign.okhttp.enabled or feign.httpclient.enabled or feign.httpclient.hc5.enabled to true, respectively, and having them on the classpath.

but it doesn't tell us which package should we depend to having these classes on the classpath.

Describe the solution you'd like

Add at least https://github.com/OpenFeign/feign to the reference page, and don't forget to add a note that users should use the same version with feign-core, or NoClassDefFoundException may thorws.

Additional context

I added a latest version of feign-okhttp to a project with spring cloud 2020.0.6, which uses feign-core 10.12 inside spring-cloud-starter-openfeign, then the unit test failed as some class cannot be found 🤣