Add a note to README.md that 'ed' is needed.

In the long run, we should get rid of that dependency by rewriting the
code that currently uses 'ed', but meanwhile this should be documented.

Sujeet Varakhedi

README.md

@@ -42,6 +42,11 @@ to the segments, and collects the results.
   * setuptools
   * epydoc
 
+   Ensure the ed text-editor is installed for gpinitsystem. For RHEL/Centos:
+   ```
+   sudo yum install ed
+   ```
+
 ## Code layout
 
 The directory layout of the repository follows the same general layout