Update README.md with link to GUIDE.MD

Author: Alathreon

README.md

@@ -24,4 +24,7 @@
 ### using IntelliJ instead of BuildJShellWrapper script
 - Open IntelliJ on the project JShellWrapper
 - Go in build → Build Artifacts... → Build
-- The jar should be located directly under JShellWrapper/out
+- The jar should be located directly under JShellWrapper/out
+
+## How to use JShellApi ?
+See [GUIDE.MD](JShellAPI/GUIDE.MD)