add welcome message

[MarianKijewski]

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PR Submission Checklist for internal contributors

• The PR Title

  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

• The PR Description

  • is free of optional paragraphs and you have filled the relevant parts to the best of your ability

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What's new in this PR?

Issues

The capabilities of the App were not clear once it's added to the conversation

Solutions

Greet, describe App's purpose and usage when added to a conversation.

Testing

Test Coverage

• I have added automated test to this contribution

How to Test

Verify that there's a message, once an App is added to the conversation.

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References

1. https://sparkbox.com/foundry/semantic_commit_messages
2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.

[MarianKijewski]