Add documentation for plugins (#454)

Author: reenberg

docs/plugins.md

@@ -0,0 +1,65 @@
+# VS Code Draw.io Integration - plugins
+
+The plugins feature lets you load Draw.io plugins, just as you can by opening
+the online version of Draw.io with the `?p=svgdata` query parameter:
+<https://www.draw.io/?p=svgdata>.
+
+Draw.io has a list of [sample plugins](https://www.drawio.com/doc/faq/plugins)
+which can be copied, or you may create your own.
+
+## Enabling a plugin in the Draw.io Integration
+
+Plugins currently needs to be loaded from an absolute path in the Draw.io
+Integration extension.  Thus for compatibility reasons (e.g., in a repository
+shared between multiple people), the plugin likely needs to be added to the
+workspace folder where your diagrams are located as well.  To facilitate this,
+the path can be specified using the `${workspaceFolder}` variable, effectively
+allowing you to specify a relative path within your workspace.
+
+Plugins are added using the `hediet.vscode-drawio.plugins` configuration
+property.  Adding this to the workspace settings makes sure that the plugin is
+automatically loaded for anyone that edits Draw.io files inside this workspace.
+
+Example:
+
+1. Download the Draw.io sample plugin `svgdata.js`, and place it in the root of
+   the workspace.
+
+1. Add the following to the workspace settings:
+
+    ```json
+    "hediet.vscode-drawio.plugins": [
+        {
+            "file": "${workspaceFolder}/svgdata.js"
+        }
+    ],
+    ```
+
+1. Open any Draw.io file
+
+1. Accept or deny loading of the plugin
+
+    If this is the first time after adding the plugin definition, or if the
+    plugin was changed, then the Draw.io Integration will show you a dialogue
+    box, asking you to allow or disallow loading of the given plugin.
+
+    What ever action you choose, is written to the
+    `hediet.vscode-drawio.knownPlugins` property, in the user settings (scope)
+    by the Draw.io Integration extension.
+
+    Your decision is explicitly only read and written to the user scope, to
+    ensure that a redistributed workspace can't load a plugin without you
+    previously having accepted the specific version of a plugin (determined
+    through the hash of the file).
+
+    Example:
+
+    ```json
+    "hediet.vscode-drawio.knownPlugins": [
+        {
+            "pluginId": "file:///full/path/to/workspace/svgdata.js",
+            "fingerprint": "<sha256>",
+            "allowed": true // or false if you disallowed it
+        }
+    ],
+    ```