update README.md

README.md

@@ -18,6 +18,15 @@ created `/etc/grid-security/xrd/*` following https://twiki.cern.ch/twiki/bin/vie
 
 ## build new image
 
+Before doing that, yum needs to be installed on the system and the file `~/.rpmmacros` needs to be created with the following content:
+
+```
+%_var /var
+%_dbpath %{_var}/lib/rpm
+```
+
+The image is build with
+
 ```
 singularity build "xcache.$(date +%F).img" xcache.singularity.def
 ```