added comments on test suite

user-doc/Installation.txt

@@ -322,6 +322,23 @@ You can check the exact version they will use by using the command
 \endverbatim
 This means that if you do not source "sourceme.sh", the tests will fails. This does not mean 
 that plumed is not working it just means that you haven't told them shell where to find plumed!
+As a related note, you can easily run the test suite using a different version of PLUMED
+(maybe an earlier version that you already installed), just making sure that it can be 
+found in the path. Clearly, if you test a given
+version of PLUMED with a test suite from a different version you can expect two
+possible kinds of innocuous errors:
+- If `plumed` executable is older than the test suite, the tests might fail since they rely on
+  some feature introduced in PLUMED in a newer version.
+- If `plumed` executable is newer than the test suite, the tests might fail since some
+  non-backward compatible change was made in PLUMED. We try to keep the number
+  of non-backward compatible changes small, but as you can see in the \ref Changelog there
+  are typically a few of them at every new major release.
+
+\attention
+Even though we regularly perform tests on [Travis-CI](http://travis-ci.org/plumed/plumed2),
+it is possible that aggressive optimizations or even architecture dependent features
+trigger bugs that did not show up on travis. So please always perform regtests when you install
+PLUMED.
 
 Notice that the compiled executable, which now sits in 'src/lib/plumed', relies
 on other resource files present in the compilation directory.