Update README to clarify integration version (#1291)
Update README to clarify integration version (#1291)

* How to find the `<INTEGRATION_VERSION>`

Clarifying how to find the `<INTEGRATION_VERSION>` for PureStorage through the CHANGELOG.md (https://github.com/DataDog/integrations-extras/blob/master/purefa/CHANGELOG.md).  

This may apply for all the integration-extras which doesn't specify in the main documentation (https://docs.datadoghq.com/agent/guide/use-community-integrations/?tab=agentv721v621).

* Update README.md

* Update README

Co-authored-by: Andrew Zhang <af2b11908f9efcc2af3e50689d77b152fbb34be0@datadoghq.com>
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