Add Mattermost

[sinansonmez]

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Types of Changes

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• New addition to list
• Fixing bug in existing item in list
• Removing item from list
• Changing structure (organization) of list

Proposed Changes

Describe each of your changes here to communicate to the maintainers why we should accept this pull request.

Mattermost is an open source platform for secure collaboration across the entire software development lifecycle.

PR Checklist

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• I have read the CONTRIBUTING guide
• I have explained why this PR is important
• I have added necessary documentation (if appropriate)

Further Comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...