Updated the Contributing to have Slack Link

[smorin]

Description

Updated the documentation to have a link to your slack channel

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/adazzle/react-data-grid/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
No link to slack

What is the new behavior?
Now has a link to slack

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[coveralls]

Changes Unknown when pulling 41f00c6 on smorin:master into ** on adazzle:master**.