on-screen-keyboard: Add info on other systems' implementations

https://phabricator.apertis.org/T8305



Signed-off-by: Ryan Gonzalez <ryan.gonzalez@collabora.com>

content/concepts/on-screen-keyboard.md

@@ -122,6 +122,24 @@ Mildenhall platform, and thus it brings several problems:
 Attempting to migrate it away from being dependent on Mildenhall would essentially amount
 to a full rewrite, resulting in little advantage versus an alternative solution.
 
+# Implementations in Other Systems
+
+Qt uses a framework-specific, client-side plugin,
+[qtvirtualkeyboard](https://doc.qt.io/qt-5/qtvirtualkeyboard-deployment-guide.html).
+Thus, like the obsolete Mildenhall speller widget mentioned previously,
+qtvirtualkeyboard is exclusively tied to Qt applications. In addition, code reuse is not
+possible, as it is under the GPLv3 license.
+
+Samsung's Tizen has a [custom IME
+framework](https://github.com/Samsung/tizen-docs/blob/master/docs/application/web/guides/text-input/input-method.md)
+built on top of HTML5 and JavaScript, as with the rest of the OS's applications. The only
+native code used is essentially for communication between the IME and the client
+application, resulting in little use for building off of Tizen's approach.
+
+LG webOS OSE uses [custom plugins](https://github.com/webosose/ime-manager) on top of the
+[Maliit IME framework](https://maliit.github.io/). Although Maliit itself is LGPL-2.1, the
+reference keyboard implementation is under LGPL-3.
+
 # New Approach
 
 A fully-fledged input method program will be a Wayland client using the *input-method*