Doc for xml.javaExtensions

Signed-off-by: azerr <azerr@redhat.com>

Author: angelozerr

README.md

@@ -80,6 +80,32 @@ Since 0.9.1:
 * `xml.validation.disallowDocTypeDecl`: Enable/disable if a fatal error is thrown if the incoming document contains a DOCTYPE declaration. Default is `false`.
 * `xml.validation.resolveExternalEntities`: Enable/disable resolve of external entities. Default is `false`.
 
+## Custom XML Extensions
+
+The [LSP4XML Language Server](https://github.com/angelozerr/lsp4xml) can be extended to support custom completion, hover, validation, rename, etc by using the Java SPI feature. vscode-xml gives the capability to start the XML language server with your custom JAR which provides your custom XML support.
+
+To do that:
+
+ * create a Java project which provides a custom XML extension providing your custom completion, hover, validation, rename, etc:
+    * create the XML extension like [MavenPlugin](https://github.com/angelozerr/lsp4xml-extensions-maven/blob/master/org.eclipse.lsp4xml.extensions.maven/src/main/java/org/eclipse/lsp4xml/extensions/maven/MavenPlugin.java).
+    * register your custom completion in the XML extension like [MavenCompletionParticipant](https://github.com/angelozerr/lsp4xml-extensions-maven/blob/master/org.eclipse.lsp4xml.extensions.maven/src/main/java/org/eclipse/lsp4xml/extensions/maven/MavenCompletionParticipant.java#L28)
+    * register your custom XML extension with SPI in the [/META-INF/services/org.eclipse.lsp4xml.services.extensions.IXMLExtension](https://github.com/angelozerr/lsp4xml-extensions-maven/blob/master/org.eclipse.lsp4xml.extensions.maven/src/main/resources/META-INF/services/org.eclipse.lsp4xml.services.extensions.IXMLExtension) file.
+    * build a JAR `your-custom-xml-extension.jar`.
+ 
+ * create a `vscode extension` which embeds the `your-custom-xml-extension.jar` JAR and declares this JAR path in the `package.json`:
+ 
+```json
+"contributes": {
+	"xml.javaExtensions": [
+		"./jar/your-custom-xml-extension"
+	]
+}
+```
+
+You can see the [vscode-xml-maven](https://github.com/angelozerr/vscode-xml-maven) sample which registers custom maven completion [MavenCompletionParticipant](https://github.com/angelozerr/lsp4xml-extensions-maven/blob/master/org.eclipse.lsp4xml.extensions.maven/src/main/java/org/eclipse/lsp4xml/extensions/maven/MavenCompletionParticipant.java#L28) for scope:
+
+![VScode XML Maven](images/vscode-xml-maven.gif)
+
 ## Contributing
 
 This is an open source project open to anyone. Contributions are extremely welcome!