TARGET_APPIMAGE (#43)

Add TARGET_APPIMAGE to AppImageSpec
as suggested by AppImageLauncher

---------

Co-authored-by: TheAssassin <[email protected]>

Author: probonopd

draft.md

@@ -23,6 +23,7 @@ The AppImage Specification is licensed under [The MIT License](https://github.co
     - [Metadata]
         + [AppStream]
         + [Update Information]
+    - [Runtime]
     - [Desktop Integration]
 
 [Introduction]: #introduction "Introduction"
@@ -247,6 +248,12 @@ You can visit https://www.pling.com/product/add to register a new product on the
 
 The __bintray-zsync__ transport was there to support [Bintray](https://bintray.com/). It is deprecated.
 
+### Runtime
+
+The AppImage runtime is the program that mounts the filesystem image and executes the payload.
+
+If the AppImage runtime is a static binary, then it **MUST** support the environment variable `TARGET_APPIMAGE`. All runtimes **SHOULD** implement this environment variable. If set to an existing path, then the AppImage Runtime will use the filesystem image from the AppImage at that path for all its operations rather than the filesystem image appended to the runtime binary.
+
 ### Desktop integration
 
 The software inside the [AppImage] **MAY** integrate into the desktop environment (e.g., by installing a [.desktop file] into the host system) on the user's behalf. However if it does so, it *SHOULD* ensure to get the explicit permission of the user, e.g., by asking for permission in a GUI window, or by reacting to a command line argument.