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Closes #328

Signed-off-by: David Thompson <[email protected]>

docs/CodeLens.md

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+
+## Code Lens
+
+### DTD features
+
+Code lens is provided in `.dtd` files to show where elements defined in the DTD are referenced.
+For instance, in the following code:
+
+```xml
+<!ELEMENT letter (to, from, title, body)>
+<!ELEMENT to (#PCDATA)>
+<!ELEMENT from (#PCDATA)>
+<!ELEMENT title (#PCDATA)>
+<!ELEMENT body (#PCDATA)>
+```
+
+There will be a code lens entry after the first line that points from the
+definition of the element `to` to where it is used in the definition of the
+element `letter`.
+
+Here is a demonstration of the above Code Lens:
+
+![A letter DTD element is declared, which consists of to, from title, and body elements. The definitions for these sub elements have a code lens above, which allows ofr navigating to the letter element declaration.](./images/code-lens/DTDElementCodeLens.gif)
+
+### XSD Features
+
+Code lens is also provided in `.xsd` files. It shows where top level types
+and elements are referenced. For instance, in the following code:
+
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<xs:schema
+    xmlns:xs="http://www.w3.org/2001/XMLSchema"
+    targetNamespace="http://example.org/my-example"
+    xmlns:NS="http://example.org/my-example">
+  <xs:element
+      name="root-element"
+      type="NS:yell" />
+  <xs:simpleType name='yell'>
+    <xs:restriction base='xs:string'>
+      <xs:pattern value='[A-Z]+!'></xs:pattern>
+    </xs:restriction>
+  </xs:simpleType>
+</xs:schema>
+```
+
+There will be a code lens above the line where the `yell` type is defined
+that points to where it is referenced when defining the `root-element` element.
+
+Here is a demonstration of the above Code Lens:
+
+![The code lens provides the number of references and links to references for an `xs:simpleType`.](./images/code-lens/XSDTypeCodeLens.gif)
+
+Here is a demonstration of the code lens for `xs:element`:
+
+![An element root is declared that contains an element boot. Above the declaration of boot, there is a code lens that links to the reference in root.](./images/code-lens/XSDElementCodeLens.gif)