Release checklist for v1.0.0

[schlessera]

The following is a list of tasks/issues we need to complete for a feature-complete v1.0.0 release of the optimizer.

• [x] New Transformer: BrowserHints [#31]
• [x] New Transformer: AutoExtensions [#32]
• [x] Copy object-position & object-fit into style attribute to reduce LCP [#6]
• [x] Server-side rendering can cause style[amp-custom] CSS overage [#19]
• [x] Support unit-based width/height attributes in ImageDimensions [#4]
• [ ] Add PHP Optimizer guide to amp.dev [#33]
• [ ] Validator: Initial implementation [#194]
• [ ] Sanitizer: Initial implementation [#195]