chore: add build instructions

BUILD.md

@@ -0,0 +1,23 @@
+# building Jelu locally
+
+## prerequisites
+
+- java 17
+- Jelu code source
+
+## building
+
+just `cd` in the root of the repo and run :
+
+```shell
+./gradlew copyWebDist && ./gradlew assemble
+```
+
+Gradlew is included in the sources so you don't need anything else.
+
+Those commands will build the frontend and then the backend including the fat jar.
+
+You will find the jar in `build/libs`, it will be named `jelu-<current-version>.jar` 
+
+The jar is the same as the one that is published in github releases.
+