Update docs to show new API.

Summary: Need API needs some new docs. I also removed the repo's `.hgignore` and added everything to fbobjc `.hgignore` via `./Tools/generate_hgignore.py` which uses Yoga's `.gitignore`.

Reviewed By: emilsjolander

Differential Revision: D4403967

fbshipit-source-id: f2158e4feb27953d1d9e21c775926e6207220c4a

docs/_docs/api/yogakit.md

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+---
+docid: yogakit
+title: YogaKit
+layout: docs
+permalink: /docs/api/yogakit/
+---
+
+> The YogaKit API is still in rapid development. Please use it and [report issues, bugs or feedback](https://github.com/facebook/yoga/issues).
+
+YogaKit is a Objective-C (and Swift-compatible) wrapper for Yoga. It allows iOS Developers to manage the layout of their views using the power of Yoga.
+
+Layout configuration is done via the [YGLayout](https://github.com/facebook/yoga/blob/master/YogaKit/YGLayout.h) object. YogaKit exposes `YGLayout` via a [category](https://developer.apple.com/library/content/documentation/General/Conceptual/DevPedia-CocoaCore/Category.html) on [UIView](https://developer.apple.com/reference/uikit/uiview).
+
+### Lifecycle
+
+As with any `UIView`, instantiate it using `-(instancetype)initWithFrame:(CGRect)frame`. YogaKit uses the frame of the parent view to determine the space that is available for children in layout.
+
+### Children
+
+Yoga relies on `UIView` subviews to build up its internal layout tree. However, a subview can exclude itself from layout by setting the `isIncludedInLayout` property on `YGLayout` to `NO`.
+
+It is also possible to query the number of children **included** in layout via `numberOfChildren`.
+
+### Layout
+To apply a layout to a view (and its' subviews) you need to call `[view.yoga applyLayout]`. This will do a layout calculation (if needed) and apply the calculated frames to every view included in the layout.
+
+In the event that you need to another layout pass on a view you can mark it dirty via `[view.yoga markDirty]`.