Add descriptions

[mattlt]

For those new to office based exercising it may be beneficial to offer explanations of the exercises.

eg.

Wrist stretch.

Stretch your arm out in front of you with the palm up. With your other hand, grab your fingers and lightly pull them down to stretch your forearm.

[craig-davis]

Here's the documentation for message attachments and formatting: https://api.slack.com/docs/attachments