#4 Chameleon: Clarify installation instructions

[cafu2500]

Chameleon from 1.2.8. only worked if a prior
installation of Chameleon was deleted manually.
E.g.
http://flux4eclipse.sourceforge.net/userguide/concepts/rational_rose/rational_rose.html
does not mention this and the installation script did
not show any warning.

Others might give up here, so better clarify the
instructions and add at least a hint to the Chameleon
installer.