added the schema reference to the list of documents for gitbook, renamed Readme.md to Summary to be consistent with conventions. (#2387)

docs/Summary.md

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+* [Introduction](./Introduction.md)
+
+## Walkthroughs
+
+* [Installing Slate](./walkthroughs/installing-slate.md)
+* [Adding Event Handlers](./walkthroughs/adding-event-handlers.md)
+* [Defining Custom Block Nodes](./walkthroughs/defining-custom-block-nodes.md)
+* [Applying Custom Formatting](./walkthroughs/applying-custom-formatting.md)
+* [Using Plugins](./walkthroughs/using-plugins.md)
+* [Saving to a Database](./walkthroughs/saving-to-a-database.md)
+* [Saving and Loading HTML Content](./walkthroughs/saving-and-loading-html-content.md)
+
+## Guides
+
+* [Commands & Queries](./guides/commands-and-queries.md)
+* [Data Model](./guides/data-model.md)
+* [Plugins](./guides/plugins.md)
+* [Rendering](./guides/rendering.md)
+* [Schemas](./guides/schemas.md)
+
+## General
+
+* [Plugins](./general/plugins.md)
+* [Resources](./general/resources.md)
+* [Contributing](../Contributing.md)
+* [Changelog](./general/changelog.md)
+* [FAQ](./general/faq.md)
+* [Glossary](./general/glossary.md)
+
+## Slate Core
+
+* [Block](./reference/slate/block.md)
+* [Commands](./reference/slate/commands.md)
+* [Data](./reference/slate/data.md)
+* [Decoration](./reference/slate/decoration.md)
+* [Document](./reference/slate/document.md)
+* [Editor](./reference/slate/editor.md)
+* [Inline](./reference/slate/inline.md)
+* [Mark](./reference//slate/mark.md)
+* [Node](./reference/slate/node.md)
+* [Operation](./reference/slate/operation.md)
+* [Plugins](./reference/slate/plugins.md)
+* [Point](./reference/slate/point.md)
+* [Range](./reference/slate/range.md)
+* [Schema](./reference/slate/schema.md)
+* [Selection](./reference/slate/selection.md)
+* [Text](./reference/slate/text.md)
+* [Utils](./reference/slate/utils.md)
+* [Value](./reference/slate/value.md)
+
+## Slate React
+
+* [Editor](./reference/slate-react/editor.md)
+* [Plugins](./reference/slate-react/plugins.md)
+* [Rendering](./reference/slate-react/rendering.md)
+* [Utils](./reference/slate-react/utils.md)
+
+## Other Packages
+
+* [`slate-html-serializer`](./reference/slate-html-serializer/index.md)
+* [`slate-hyperscript`](./reference/slate-hyperscript/index.md)
+* [`slate-plain-serializer`](./reference/slate-plain-serializer/index.md)
+* [`slate-prop-types`](./reference/slate-prop-types/index.md)