Make it easier to detect the ast-redux Rust lib

Summary:
You now need to do something like this in order to have CMake find the ast-redux lib:

export CMAKE_PREFIX_PATH=/path/to/ast-redux/target/debug:$CMAKE_PREFIX_PATH
cmake .

Also updated README

Reviewers: egospodinova

Reviewed By: egospodinova

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D6618