Adding instructions how to use ral in ROOT REPL

README.md

@@ -1,2 +1,42 @@
 # ral
-ROOT DataFrame Analysis Repository
+
+Key4hep library for use in ROOT RDataFrame based analysis.
+
+## Interactive use
+
+In order to use this library inside ROOT REPL one can open a root file with
+```
+root test.root
+```
+and then in the ROOT REPL do
+```cpp
+gSystem->Load("libral.so");
+loadRal();
+ROOT::RDataFrame dframe(*events);
+auto hist = dframe.Define("pT", "k4::ral::ReconstructedParticle::get_pt(ReconstructedParticles)").Histo1D("pT");
+hist->Draw();
+```
+
+The whole output of the ROOT REPL can be seen here:
+```
+   ------------------------------------------------------------------
+  | Welcome to ROOT 6.32.04                        https://root.cern |
+  | (c) 1995-2024, The ROOT Team; conception: R. Brun, F. Rademakers |
+  | Built for linuxx8664gcc on Aug 21 2024, 03:51:07                 |
+  | From heads/master@tags/v6-32-04                                  |
+  | With g++ (GCC) 11.4.1 20230605 (Red Hat 11.4.1-2)                |
+  | Try '.help'/'.?', '.demo', '.license', '.credits', '.quit'/'.q'  |
+   ------------------------------------------------------------------
+
+root [0]
+Attaching file test.root as _file0...
+(TFile *) 0x23c5ce0
+root [1] gSystem->Load("libral.so");
+root [2] loadRal()
+(bool) true
+root [3] ROOT::RDataFrame dframe(*events);
+root [4] auto hist = dframe.Define("pT", "k4::ral::ReconstructedParticle::get_pt(ReconstructedParticles)").Histo1D("pT");
+root [5] hist->Draw();
+Info in <TCanvas::MakeDefCanvas>:  created default TCanvas with name c1
+root [6]
+```