Update 'Selection code'

Selection-code.md

@@ -43,6 +43,12 @@ gROOT->ProcessLine(".L path/to/your/folder/B2KstarMuMu/Code/Selection/Design.cpp
 }
 ```
 
+And we need to tell ROOT where to look: create a '.rootrc' file in your home directory with:
+```
+Rint.Load:  $(HOME)/.rootalias.C
+```
+
+
 Anytime you start ROOT by using `root`, the intro printout shows up. I find it useful to set an alias to automatically include the `-l`flag that supresses this output. Open your `~.bashrc` file and add
 ```
 alias root="root -l"