add flask_zipkin into community_instrumentations.yml

[killpanda]

We created a new library, and it has already been using in our production environment for a while.

It works fine to us.

[codefromthecrypt]

cool! who's the "we" that's using this in production (curious)

[killpanda]

@adrancole thanks for merging! Currently we work in a company called VIPKID. Zipkin is a great tool.

[killpanda]

@adriancole sorry for typo

[codefromthecrypt]

:thumbsup: