update readme

update makefile and readme

Makefile

@@ -29,7 +29,8 @@ all: build/imgui.o $(ROOT_IFACE) ubsan
 clean:
 	rm build/*.o rntviewer $(ROOT_IFACE)
 
-build/imgui.o: src/imgui_inc.cpp third_party/imgui/imgui.h
+build/imgui.o: src/imgui_inc.cpp
+	@mkdir -p $(@D)/build
 	$(CXX) -O3 -fPIC -c -Ithird_party/imgui $< -o $@
 
 $(ROOT_IFACE): src/root/RMicroFileReader.cxx

README.md

@@ -15,6 +15,7 @@ rntviewer currently only works on Linux (PRs welcome).
 The dependencies are:
   - GLFW3 (`libglfw3-dev` on debian, `glfw-devel` on fedora, `glfw` on arch)
   - ROOT (likely bleeding edge version, compiled from sources)
+  - libxxhash3-dev
 
 Other than those, rntviewer depends on Dear ImGui and GLAD (both included in the source tree).
 
@@ -22,6 +23,15 @@ The machine must support OpenGL 4.
 
 ### Building
 Building is done with `make`. Make sure to `source /path/to/root/bin/thisroot.sh` before running it.
+You probably want to build the release version:
+```
+$ make r
+```
+
+You can test that the tool is working properly with:
+```
+$ ./rntviewer ntpl lmext1.root
+```
 
 ### Usage
 ```