Provide hints about ENTER, SHIFT+ENTER during chat entry

[JasonWeill]

Problem

Especially to users more familiar with other chat programs, it is not safe to assume that a user will know to press SHIFT+ENTER to send a message, as ENTER adds a new line.

Proposed Solution

When the user has typed anything into the chat UI text box, provide a hint below the text box stating that SHIFT+ENTER will send the message, and/or that ENTER adds a new line.

Additional context

When I have typed information into Slack's message box, using default settings on macOS, I see "Shift + Return to add a new line" below the text box: