fix swagger document (gorse-io#478)

fix insert duplicate users/items/feedback (gorse-io#465)

return empty list instead of nil (gorse-io#456)

fix wrong metrics (gorse-io#446)

add CONTRIBUTING.md (gorse-io#424)

allow clock error in the cluster (gorse-io#397)

return zero recall on empty ground truth (gorse-io#381)

evaluate HNSW in parallel (gorse-io#371)

optimize fallback item-based recommender (gorse-io#350)

optimize docker image version (gorse-io#335)