add docs, update readme

Readme.md

@@ -58,7 +58,7 @@ If you're using Slate for the first time, check out the [Getting Started](./docs
   - Customizing Slate's Behavior
   - Adding Plugins
 
-- **Core Concepts**
+- [**Core Concepts**](docs/concepts.md)
   - Statelessness & Immutability
   - [The Document Model](docs/getting-started.md#the-document-model)
   - The Selection Model

docs/concepts.md

@@ -0,0 +1,44 @@
+
+This document explain the core concepts that Slate is built on around. It is helpful to read it through in it's entirety to get a good mental model for how Slate works.
+
+- [Statelessness & Immutability](#statelessness-immutability)
+- [The Document Model](#the-document-model)
+- [The Selection Mode](#the-selection-model)
+- [Transforms](#transforms)
+- [Plugins](#plugins)
+
+
+### Statelessness & Immutability
+
+- all the data is immutable, for performance
+- changes propagated up through the single `onChange` handler
+
+
+### The Document Model
+
+- recursive, nested tree 
+- document, blocks, and inlines implement node interface
+- blocks and inlines can be nested to any depth
+- blocks contain inlines and text
+- inlines contain text
+- always a text node at the leaves, for selection handling
+- void nodes can't have content, but still have an empty text node
+
+
+### The Selection Model
+
+- selection is always relative to text nodes (is normalized)
+- "blocks" always refers to the closest block parent
+
+
+### Transforms
+
+- no updating of document/selection outside of transforms
+- wrap blocks wraps close to the text
+- wrap inline wraps far from the text
+
+
+### Plugins
+
+- everything is a plugin, even core is one
+- the editor's props become the highest-priority plugin