Tweak extensions page texts (#1550)

Signed-off-by: Jari Kolehmainen <jari.kolehmainen@gmail.com>

docs/extensions/usage/README.md

@@ -1,3 +1,23 @@
 # Using Extensions
 
-TBD
+The features that Lens includes out-of-the-box are just the start.
+Lens extensions let you add new features to your installation to support your workflow.
+Rich extensibility model lets extension authors plug directly into the Lens UI and contribute functionality through the same APIs used by Lens itself.
+
+![Extensions](images/extensions.png)
+
+## Installing an Extension
+
+You can install a dowloaded extension .tgz package by going to **File** > **Extensions** (**Lens** > **Extensions** on Mac). Alternatively you can point an URL to .tgz file. An installed extension is enabled automatically.
+
+## Enabling an Extension
+
+Go to **File** > **Extensions** (**Lens** > **Extensions** on Mac) and click "Enable" button.
+
+## Disabling an Extension
+
+Go to **File** > **Extensions** (**Lens** > **Extensions** on Mac) and click "Disable" button.
+
+## Uninstalling an Extension
+
+Go to **File** > **Extensions** (**Lens** > **Extensions** on Mac) and click "Uninstall" button.