chore: add changelog entry, extension point documentation

CHANGELOG.md

@@ -1,6 +1,10 @@
 # Snyk Changelog
 
-## [2.6.1]
+## [2.7.0]
+### Added
+- Snyk controller extension point
+
+## [2.6.2]
 ### Fixed
 - remove more intellij-specific code and configuration
 

EXTENSIONS.md

@@ -0,0 +1,83 @@
+# Snyk Extensions
+
+This plugin offers an extension point for integrating Snyk with other Jetbrains IDE Plugins.
+
+## What is the Snyk Controller?
+
+The Snyk Controller is, put simply, a way to control Snyk from other plugins.
+
+## Why would I use the Snyk Controller extension?
+
+There are a few of use cases for the Snyk controller:
+
+### Initiating a Snyk scan
+
+There may be situations in which a plugin wants to initiate a security scan, especially at the end of a workflow which
+introduces changes to the project source code, manifest dependencies, OCI container builds, infrastructure files,
+etc. -- anything Snyk can scan.
+
+### Determining whether Snyk is authenticated to a user
+
+It might be useful to know whether the user has authenticated with Snyk; is Snyk ready to run scans?
+
+## How do I use the extension in my plugin?
+
+### Add a dependency on this plugin to your plugin's `plugin.xml` file.
+
+Release >= 2.7.0 provides the extension point.
+
+```xml
+<depends>io.snyk.snyk-intellij-plugin</depends>
+```
+
+### Declare the extension
+
+Add an extension for `io.snyk.snyk-intellij-plugin.controllerLifecycleManager` to your plugin's `plugin.xml` file.
+
+```xml
+<extensions defaultExtensionNs="io.snyk.snyk-intellij-plugin">
+  <controllerLifecycleManager implementation="com.example.demointellij.MySnykControllerLifecycleManager"/>
+</extensions>
+```
+
+### Implement the lifecycle manager interface
+
+The lifecycle manager will be instantiated and passed an instance of the controller when the Snyk plugin is initialized.
+
+```kotlin
+package com.example.demointellij
+
+import io.snyk.plugin.extensions.SnykController
+import io.snyk.plugin.extensions.SnykControllerLifecycleManager
+
+class HelloWorldControllerLifecycleManager : SnykControllerLifecycleManager {
+  override fun register(controller: SnykController) {
+    Utils().getSnykControllerService().setController(controller)
+  }
+}
+```
+
+Snyk recommends building a [service](https://plugins.jetbrains.com/docs/intellij/plugin-services.html) to receive the
+controller instance and provide it to other parts of your plugin.
+
+### Use the controller in your plugin
+
+See [SnykController](https://github.com/snyk/snyk-intellij-plugin/blob/main/src/main/kotlin/io/snyk/plugin/extensions/SnykController.kt)
+for the current Snyk methods supported.
+
+#### Initiating a scan
+
+```kotlin
+Utils().getSnykControllerService().getController()?.scan()
+```
+
+## What compatibility guarantees are there, for consumers of this extension?
+
+Our [semantic version](https://semver.org/) releases indicate the compatibility of the extension API with respect to
+past releases.
+
+With a minor version increment, new methods may be added to interfaces, but existing methods will not be removed or
+their prototypes changed.
+
+With a major version increment, there are no compatibility guarantees. Snyk suggests checking the release notes, source
+changes, and compatibility testing before upgrading.