Added note on dependency for centos install

Author: rhoerbe

doc/install.rst

@@ -44,6 +44,10 @@ Once you have installed all the necessary prerequisites a simple::
 
 will install the basic code.
 
+Note for rhel/centos 6: cffi depends on libffi-devel to compile
+So you might want first to do:
+yum install libffi-devel
+
 After this you ought to be able to run the tests without an hitch.
 The tests are based on the pypy test environment, so::