Add comment to clarify change

src/Composer/Repository/VcsRepository.php

@@ -46,6 +46,7 @@ class VcsRepository extends ArrayRepository
             'git'           => 'Composer\Repository\Vcs\GitDriver',
             'hg-bitbucket'  => 'Composer\Repository\Vcs\HgBitbucketDriver',
             'hg'            => 'Composer\Repository\Vcs\HgDriver',
+            // svn must be last because identifying a subversion server for sure is practically impossible
             'svn'           => 'Composer\Repository\Vcs\SvnDriver',
         );