kate/README: add a minimal list of build dependencies

Summary: Using this list the compile time can be reduced to a minimum when rebuilding Kate.

Reviewers: #kate, ngraham, cullmann

Reviewed By: #kate, ngraham, cullmann

Subscribers: cullmann, ngraham, kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D18082