Add project's README

Signed-off-by: Iltotore <rafbodaha@gmail.com>

README.md

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+# SpigotScala
+Extends the Spigot API using Scala.
+
+# Description
+SpigotScala is a Scala-specific rework of the Spigot API. It includes syntactic sugar and scala-specific features.
+
+# Features
+SpigotScala includes:
+- Scala plugin wrapper as a Spigot plugin
+- [Factory methods sugaring using the `apply` method](https://github.com/Iltotore/SpigotScala/wiki/Factory-methods)
+- [Method aliases (like operators for arithmetic methods)](https://github.com/Iltotore/SpigotScala/wiki/Decorators)
+- [Better generic support and removal of class parameters](https://github.com/Iltotore/SpigotScala/wiki/Decorators#better-generics)
+- [Usage of Scala implicits to extend existing classes](https://github.com/Iltotore/SpigotScala/wiki/Decorators)
+- [Custom DSLs](https://github.com/Iltotore/SpigotScala/wiki/DSL)
+
+# Installation
+## Developers
+You can add the [release jar](https://github.com/Iltotore/SpigotScala/releases) to your project libraries
+or use a build tool like Gradle or SBT:
+
+<details>
+<summary>Using Gradle</summary>
+
+```gradle
+repositories {
+  mavenCentral()
+}
+
+dependencies {
+  implementation 'io.github.iltotore:spigot-scala_2.13:version'
+}
+```
+</details>
+
+<details>
+<summary>Using SBT</summary>
+
+```sbt
+libraryDependencies += "io.github.iltotore" %% "spigot-scala" % "version"
+```
+</details>
+
+## Server owners
+You must add the [release jar](https://github.com/Iltotore/SpigotScala/releases) to your `plugins` directory, then restart/reload the server.