Add documentation for Sidekiq

[mwear]

Add documentation for the Sidekiq instrumentation adapter. The implementation exists here, but we should have a readme describing its behavior and options.

[AzfaarQureshi]

Hi! Am I free to pick this up? The link (https:/open-telemetry/opentelemetry-ruby/tree/master/adapters/sidekiq) seems to be broken though, did the implementation get refactored somewhere else? cc - @alolita

[ericmustin]

@AzfaarQureshi sounds good, I've assigned to you. the repo structure changed slightly since this issue was filed, it can be found here now https:/open-telemetry/opentelemetry-ruby/tree/master/instrumentation/sidekiq