Add gradient instructions to icons HIG

### Expected behavior

There is a simple instruction/guideline to add a small gradient to the icon after adding a border and drop shadow.

### Actual behavior

A gradient is shown on the system icons, but there is no guideline/instruction for it.

### Steps to reproduce the behavior

1. Go to the Human Interface Guidelines.
2. Go to "Iconography."
3. Can't find a spec on gradients.

---
Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/47129190-add-gradient-instructions-to-icons-hig?utm_campaign=plugin&utm_content=tracker%2F10548672&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F10548672&utm_medium=issues&utm_source=github).