added slack guidelines file (#1138)
added slack guidelines file (#1138)

* added slack guielines

* Update SLACK_GUIDELINES.md

* Update SLACK_GUIDELINES.md

* Update SLACK_GUIDELINES.md

* Update SLACK_GUIDELINES.md

* Update SLACK_GUIDELINES.md

Co-authored-by: Taylor Miller <35403485733bfcfcf26aff08d3820296c75e4139@users.noreply.github.com>
great-expectations/great_expectations,great-expectations/great_expectations,great-expectations/great_expectations,great-expectations/great_expectations