Add documentation where to find Monocle source code

src/docs/source-code.md

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+# Monocle Source
+
+It is often difficult to remember where to find the Monocle source code in 
+order to build it. This document notes where to find the Monocle source code
+for Java LTS versions.
+
+In all cases, the code for Monocle is found in the following folder in the source archive:
+
+`modules/javafx.graphics/src/main/java/com/sun/glass/ui/monocle`
+
+## JFX 21
+Since JFX 14 all JFX source code is now on [Github](#github)
+
+URL: https://github.com/openjdk/jfx/tree/jfx21
+
+## JFX 17
+
+Since JFX 14 all JFX source code is now on [Github](#github)
+
+URL: https://github.com/openjdk/jfx/tree/jfx17
+
+## JFX 11
+
+Version 11.0.2 was the last freely supported version of JFX 11. All other 
+releases require an LTS support contract with Gluon.
+
+Download 11.0.2 from [Gluon](#gluon): https://download2.gluonhq.com/openjfx/11.0.2/openjfx-11.0.2_linux-x64_bin-sdk.zip
+
+
+## JFX 8
+JavaFX 8 source code is bundled with any JDK 8 release. 
+
+URL: https://adoptium.net/temurin/releases/?os=linux&arch=x64&package=jdk&version=8
+
+## GitHub
+1. Navigate to: https://github.com/openjdk/jfx
+1. Select the branch for the version of JFX for which you are building Monocle
+1. Click the `<> Code` button
+1. Click the `Download ZIP` link
+
+## Gluon
+1. Navigate to: https://gluonhq.com/products/javafx/
+1. Scroll down to `Downloads` section
+1. Select the `Include older versions` checkbox
+1. Choose the version of JFX for which you are building Monocle
+1. Click the `Download` button
+
+## Eclipse Temurin
+1. Navigate to: https://adoptium.net/temurin/releases
+1. Select the version of Java for which you are building Monocle
+1. Click the archive link to download