Add detailed documentation on award emojis

Currently we have http://doc.gitlab.com/ce/workflow/gitlab_flow.html#award-emojis-on-issues-and-merge-requests

Some questions to be answered:

• Why should one use the award emojis? What are the use cases?
• Are they counted as comments?
• Can I sort the issues/MRs based on emoji count?
• Is there an API for award emojis? Namely, can one find the number of award emojis with an API query?
• What are the differences between thumbs(up|down) emojis and all the other ones?