Merge pull request #232 from mattrose/issue-227-install

fix INSTALL instructions for setuptools

INSTALL.md

@@ -39,7 +39,7 @@ And go from there.  Manpages are available in the 'doc' directory.
 To install properly, run:
 
     python3 setup.py build
-    python3 setup.py install --record=install-files.txt
+    python3 setup.py install --single-version-externally-managed --record=install-files.txt
 
 See `--help` for an overview of the available options; e.g. `--prefix` to
 install to a custom base directory, and `--without-gettext` to avoid