Update sdk.md

reference_guide/project_model/sdk.md

@@ -39,3 +39,68 @@ Sdk projectSdk = ProjectRootManager.getInstance(project).getProjectSdk();
   ```
   
 See the following [code sample](https://github.com/JetBrains/intellij-sdk-docs/blob/master/code_samples/project_model/src/com/intellij/tutorials/project/model/ProjectSdkAction.java) to get more familiar with SDK manipulation tool set.
+
+## Working with your own SDK
+
+To create your own SDK, You need to create a class extends `SdkType`, and set your recommended SDK name, SDK home path, a presentable name. <br/>
+Finally you need to implement `isValidSdkHome` to do the SDK validation, and this example just check if the given path is a directory.
+
+```java
+import com.intellij.openapi.projectRoots.*;
+
+public class MySdkType extends SdkType {
+    public MySdkType() {
+        super("MY_SDK_NAME");
+    }
+
+    @Override public @Nullable String suggestHomePath() {
+        return "/usr/share/covscript";
+    }
+
+    @Override public boolean isValidSdkHome(@NotNull String path) {
+        return Files.isDirectory(Paths.get(path));
+    }
+
+    @Override public @Nullable String suggestSdkName(@NotNull String s, @NotNull String s1) {
+        return "Replace with your SDK name";
+    }
+
+    @Override public @Nullable AdditionalDataConfigurable createAdditionalDataConfigurable(
+            @NotNull SdkModel sdkModel, @NotNull SdkModificator sdkModificator) {
+        return null; // not needed
+    }
+
+    @Override public @NotNull String getPresentableName() {
+        return "My Awesome Sdk";
+    }
+
+    @Override public void saveAdditionalData(
+            @NotNull SdkAdditionalData sdkAdditionalData, @NotNull Element element) {
+        // leave blank
+    }
+
+    @Override public boolean setupSdkPaths(@NotNull Sdk sdk, @NotNull SdkModel sdkModel) {
+        SdkModificator modificator = sdk.getSdkModificator();
+        modificator.setVersionString(getVersionString(sdk));
+        modificator.commitChanges();
+        return true;
+    }
+}
+```
+
+Optionally, you can override `getIcon` to set an icon to your SDK type.
+
+The last method is not abstract, so it's possible to remove it. But to save your SDK settings
+(say, the next time you open your IDE, the SDK will still be there), you should override it.
+
+To let users create or select an existing SDK, you may do this in some configuration panels (like, module wizard, facet, or run configuration):
+
+```java
+ProjectJdksEditor editor = new ProjectJdksEditor(sdkCurrentlyUsed, project, parentGuiComponent);
+editor.setTitle("Select a SDK");
+editor.show();
+if (editor.isOK()) {
+    Sdk newSdkSelected = editor.getSelectedJdk();
+    // do your update job here
+}
+```