Add build introduction

BUILD.md

@@ -0,0 +1,11 @@
+Here is a simple introduction lead you to build this project
+
+## Build NMS Dependencies
+
+Mojang does not allow anyone to publish the remapped NMS jar to any public repository,
+so you need to build it yourself
+
+1. Download [BuildTools](https://www.spigotmc.org/wiki/buildtools/)
+2. execute `java -jar BuildTools.jar --rev 1.21 --remapped` to install NMS 1.21 to your local repository
+
+

README.md

@@ -177,3 +177,7 @@ self-commands:
   - '/login abc123!'
 ```
 
+# Build Project
+See the [introduction](./BUILD.md).
+
+