added documentation for plugins system

resources/plugins/doc/plugins_readme.md

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+# LUA Plugin system
+
+This plugin system was made for injecting LUA scripts in dcs-liberation missions.
+
+The resources for the plugins are stored in the `resources/plugins` folder ; each plugin has its own folder.
+
+## Standard plugins
+
+### The *base* plugin
+
+The *base* plugin contains the scripts that are going to be injected in every dcs-liberation missions.
+It is mandatory.
+
+### The *JTACAutolase* plugin
+
+This plugin replaces the vanilla JTAC functionality in dcs-liberation.
+
+### The *VEAF framework* plugin
+
+When enabled, this plugin will inject and configure the VEAF Framework scripts in the mission.
+
+These scripts add a lot of runtime functionalities :
+
+- spawning of units and groups (and portable TACANs)
+- air-to-ground missions 
+- air-to-air missions
+- transport missions
+- carrier operations (not Moose)
+- tanker move
+- weather and ATC
+- shelling a zone, lighting it up
+- managing assets (tankers, awacs, aircraft carriers) : getting info, state, respawning them if needed
+- managing named points (position, info, ATC)
+- managing a dynamic radio menu
+- managing remote calls to the mission through NIOD (RPC) and SLMOD (LUA sockets)
+- managing security (not allowing everyone to do every action)
+- define groups templates
+
+For more information, please visit the [VEAF Framework documentation site](https://veaf.github.io/VEAF-Mission-Creation-Tools/) (work in progress)
+
+## Custom plugins
+
+Custom scripts can also be injected by dropping them in the `resources/plugins/custom` folder, and writing a `__plugins.lst` file listing them in order.
+See the `__plugins.lst.sample` file for an example.
+
+## New settings pages
+
+![New settings pages](0.png "New settings pages")
+
+Custom plugins can be enabled or disabled in the new *LUA Plugins* settings page.
+
+![LUA Plugins settings page](1.png "LUA Plugins settings page")
+
+For plugins which expose specific options (such as "use smoke" for the *JTACAutoLase* plugin), the *LUA Plugins Options* settings page lists these options.
+
+![LUA Plugins Options settings page](2.png "LUA Plugins settings page")
+
+