Add a tip for installation in README (cysignals dependance)

Author: vinklein

README.rst

@@ -42,6 +42,14 @@ If you want to try the development version use
 
     $ pip install git+https://github.com/defeo/cypari2.git [--user]
 
+If you have an error saying libpari-gmp*.so* is missing and have all requirements
+already installed, try to reinstall cysignals and cypari2
+
+::
+
+    $ pip install cysignals --upgrade [--user]
+    $ pip install cypari2 --upgrade [--user]
+
 Other
 ^^^^^