Created Project Architecture (markdown)

Project-Architecture.md

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+![Plan Header](https://puu.sh/AXSg7/5f2f78c06c.jpg)
+
+# Project Architecture (How the plugin is put together)
+
+This page attempts to help new developers start contributing to Plan.
+
+It does not go over the build tool & commands for building as those are detailed in [[Project Setup]].
+
+Page version: **5.0 build 263** [Commit](https://github.com/plan-player-analytics/Plan/commit/f061d4007990be811f2bf622abe23da815d3a1fb)
+
+### Table of contents
+
+- Java
+  - Modules & Dependencies between modules
+  - Dagger & Dependency injection
+  - Structure outline of `common`-module
+  - About Platform APIs
+- Web dev
+  - Technology stack
+  - Template-like html
+  - Static Resources
+
+# Modules & Dependencies between modules
+
+Module | Role
+-- | --
+`api` | Contains code for [public API](https://github.com/plan-player-analytics/Plan/wiki/APIv5) interface classes
+`extensions` | Contains built-in extensions that use DataExtension API (in public API)
+`common` | Main package for everything
+`bukkit`, `bungee`, `sponge`, `velocity` | Platform specific modules
+`plugin` | Creates a single jar out of the modules
+
+Most of the time work is done in `common` module as platforms are abstracted away.
+
+## Build order
+
+`api` -> `extensions` -> `common` -> `bukkit`, `bungee`, `sponge`, `velocity` -> `plugin`
+
+From the build order, the dependencies between the modules should be apparent.
+
+# Dagger & Dependency Injection
+
+Dagger (https://dagger.dev/) is used extensively around the project.
+It was used as a remedy to reduce static usage. (*See decisions here [Refactor plugin to use static getters](https://github.com/plan-player-analytics/Plan/wiki/Architectural-Decision-Records#411---417-3rd-dec-2017---2nd-march-2018) and [Dependency Injection: Dagger](https://github.com/plan-player-analytics/Plan/wiki/Architectural-Decision-Records#450-11-august-2018---27-october-2018)*)
+
+`@Singleton` annotations are used to tell Dagger that only a single instance should exist per initialized Component (So that each `PlanSystem` only has one `DatabaseSystem` instead of returning a new one every time the database is needed.)
+
+`@Inject` annotations are used in Constructors to tell Dagger to resolve the dependencies for constructing a new instance of this class.
+
+Note that the Inject constructors allow Injecting an instance of the class the constructor is in to another object:
+```
+public class Foo {
+    @Inject
+    public Foo() {}
+}
+
+public class Bar {
+    @Inject
+    public Bar(Foo foo) {}
+}
+```
+This is the main mechanism used for instantiating different system level objects, more about those below.
+
+# Structure outline of `common`-module
+
+*Under construction*