Add README

Author: Gegy

README.md

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+# Fantasy
+Fantasy is a library that allows for dimensions to be created and destroyed at runtime on the server.
+It supports both temporary dimensions which do not get saved, as well as persistent dimensions which can be safely used across server restarts.
+
+## Using
+
+### Adding to Gradle
+To add Fantasy to your Gradle project, add the Nucleoid Maven repository and Fantasy dependency.
+`FANTASY_VERSION` should be replaced with the latest version from [Maven](https://maven.nucleoid.xyz/xyz/nucleoid/fantasy).
+```gradle
+repositories {
+  maven { url = 'https://maven.nucleoid.xyz/' }
+}
+
+dependencies {
+  // ...
+  modImplementation 'xyz.nucleoid:fantasy:FANTASY_VERSION'
+}
+```
+
+### Creating Runtime Dimensions
+All access to Fantasy's APIs goes through the `Fantasy` object, which can be acquired given a `MinecraftServer` instance.
+
+```java
+Fantasy fantasy = Fantasy.get(server);
+// ...
+```
+
+All dimensions created with Fantasy must be set up through a `RuntimeWorldConfig`.
+This specifies how the dimension should be created, involving a dimension type, seed, chunk generator, and so on.
+
+For example, we could create a config like such:
+```java
+RuntimeWorldConfig worldConfig = new RuntimeWorldConfig()
+        .setDimensionType(DimensionType.OVERWORLD_REGISTRY_KEY)
+        .setDifficulty(Difficulty.HARD)
+        .setGameRule(GameRules.DO_DAYLIGHT_CYCLE, false)
+        .setGenerator(server.getOverworld().getChunkManager().getChunkGenerator())
+        .setSeed(1234L);
+```
+
+Values such as difficulty, game rules, and weather can all be configured per-world. 
+
+#### Creating a temporary dimension
+Once we have a runtime world config, creating a temporary dimension is simple:
+```java
+CompletableFuture<RuntimeWorldHandle> future = fantasy.openTemporaryWorld(worldConfig);
+future.thenAccept(worldHandle -> {
+    // when the temporary world is created, set a block
+    ServerWorld world = worldHandle.asWorld();
+    world.setBlockState(BlockPos.ORIGIN, Blocks.STONE.getDefaultState());
+
+    // we don't need the world anymore, delete it!
+    worldHandle.delete();
+});
+```
+Explicit deletion is not strictly required for temporary worlds: they will be automatically cleaned up when the server exits.
+However, it is generally a good idea to delete old worlds if they're not in use anymore.
+
+#### Creating a persistent dimension 
+Persistent dimensions work along very similar lines to temporary dimensions:
+
+```java
+CompletableFuture<RuntimeWorldHandle> future = fantasy.getOrOpenPersistentWorld(new Identifier("foo", "bar"), config);
+future.thenAccept(worldHandle -> {
+    // when the persistent world is created, set a block
+    ServerWorld world = worldHandle.asWorld();
+    world.setBlockState(BlockPos.ORIGIN, Blocks.STONE.getDefaultState());
+});
+```
+
+The main difference involves the addition of an `Identifier` parameter which much be specified to name your dimension uniquely.
+
+Another **very important note** with persistent dimensions is that `getOrOpenPersistentWorld` must be called to re-initialize
+the dimension after a game restart! Fantasy will not restore the dimension by itself- it only makes sure that the world data
+sticks around. This means, if you have a custom persistent dimension, you need to keep track of it and all its needed
+data such that it can be reconstructed by calling `getOrOpenPersistentWorld` again with the same identifier.