Document retry mechanic in a more detailed way

[mschwitalla]

The retry mechanic of riptide is only briefly described in the documentation and could have more details to cover different questions.

Detailed Description

Especially the note that only idempotent requests are retried should be documented as this cost me quite some time to figure out. Additionally you could document that by default only connection errors are retried and otherwise you should define the retry behavior yourself. The riptide-failsaife documentation reads like the configuration of the retry behavior in the config would be good enough.

Context

It will prevent people from making mistakes that could be prevented with a couple of sentences in the documentation.

[fatroom]

Idempotent requests behaviour clearly stated in readme: https://github.com/zalando/riptide/tree/master/riptide-failsafe#safe-and-idempotent-methods

Will add the notes about what actually retried as well.