📖 Missing guide: Forms in AMP

## 📖 Missing or out-of-date documentation

**Describe the content that is missing or should be up dated**
We should have a guide that describes how to do forms well in AMP, w/ form validation, auto complete etc. and outlines all available options.

**Which part of the site would that content live at?**
Guides and tutorials.

**What's the type of content?**
Probably a guide, but could be complemented with a tutorial.