Add ability to specify defaults for variables

docs/custom_gadget_file.md

@@ -0,0 +1,25 @@
+---
+title: Configuration
+---
+
+This document describes how to use vimspector's `install_gadget.py` to install
+custom debug adapters. This can be useful as a way to get an adapter working
+that isn't officially supported by Vimspector, but otherwise can be made to work
+by simply downloading the VScode extension into the gadget directory.
+
+## Usage
+
+```
+./install_gadget.py --enable-custom=/path/to/a.json \
+                    --enable-custom=/path/to/b.json`
+```
+
+This tells `install_gadget.py` to read `a.json` and `b.json` as _gadget
+definitions_ and download/unpack the specified gadgets into the gadget dir, just
+like the supported adapters.
+
+## Gadget Definitions
+
+A _gadget definition_ is a file containing a single JSON object definition,
+describing the debug adapter and how to download and install it. This mechanism
+is crude but can be effective.