Add docs, refs #9422

src/Composer/Installer/InstallerInterface.php

@@ -73,7 +73,7 @@ interface InstallerInterface
      * Returns the installation path of a package
      *
      * @param  PackageInterface $package
-     * @return string           path
+     * @return string           path to install to, which MUST not end with a slash
      */
     public function getInstallPath(PackageInterface $package);
 }