Add overview of the APIs exported by @webdoc/model

Author: ShukantPal

packages/webdoc-model/README.md

@@ -1,15 +1,64 @@
 # @webdoc/model
 
-This package is the API to manipulating loaded documentation tree models. [@webdoc/types](https://github.com/webdoc-js/webdoc/tree/master/packages/webdoc-types) defines the node structure.
+This package is the API for editing and querying in-memory documentation tree models.  [@webdoc/types](/packages/webdoc-types) defines the node structure.
 
 ## Installation :package:
 
+This package is usually a dependency of templates and other webdoc packages.
+
 ```bash
 npm install @webdoc/model
 ```
 
-## Usage
+## Usage :newspaper_roll:
+
+```js
+import * as model from '@webdoc/model';
+```
+
+### Constructing and editing documents
+
+```js
+const entity = model.createDoc(
+  "DocumentedEntity",
+  "ClassDoc",
+  {
+    "description": "This is a programmatically created document!",
+  }
+);
+
+const entityNs = model.doc("Documentation.Entities", documentTree);
+
+model.addChildDoc(entity, entityNs);
+```
+
+@webdoc/model exports helper functions for creating and searching documents and mounting them into document trees.
+
+### Data types
 
 ```js
-const Model = require('@webdoc/model');
+model.createFunctionType(
+  [model.createSimpleKeywordType("Promise")], // params
+  model.createSimpleKeywordType("boolean"),   // returns
+);
 ```
+
+@webdoc/model exports helper functions for creating and joining data types. The `DataType` type is defined in [@webdoc/types](/packages/webdoc-types).
+
+### Querying
+
+```js
+// Gets all the methods named "generic" in DocumentedEntity. Each signature of the method has a separate document. The
+// # operator excludes any static "generic"-named methods.
+const genericSignatures = model.query("Documentation.Entities.DocumentedEntity#generic", documentTree);
+```
+
+@webdoc/model exports a query engine for its document path language.
+
+### Runtime type-checking
+
+```js
+const isInstantiable = model.isClass(doc) || model.isInterface(doc);
+```
+
+@webdoc/model exports helper functions for checking the types of document passed.