Update setting_up_environment.md

docs/using_eessi/setting_up_environment.md

@@ -5,6 +5,12 @@ To set up the EESSI environment, you can either load an EESSI module with Lmod o
 ## Loading an EESSI module with Lmod
 
 There are a few different scenarios where you may want to set up the EESSI environment by loading an EESSI module with Lmod:
+    !!! note "Why do we recommend to unset `MODULEPATH`?"
+        Unsetting `MODULEPATH` may not be possible at some sites, and some compromise or workaround may
+        be necessary. The underlying reason to suggest this is that EESSI and your system are most likely based on
+        two different operating system distributions - EESSI uses Gentoo Prefix, your system almost certainly uses some other
+        distribution. If you can find a way to ensure that the _software stacks_ from your site and EESSI do not mix
+        (in particular when someone is building new software!), then this should be good enough.
 
 1.  You are already using Lmod with version >= 8.6