Fix CMakeLists always building examples.
Summary: The README.md states that KGAPI_BUILD_EXAMPLES can be used to build examples, however examples are build regardless of the variables value. This patch fixes this behavior and adds an explanation for the BUILD_TESTING variable.
Reviewers: dvratil
Reviewed By: dvratil
Subscribers: kde-pim, LibKGAPI
Differential Revision: https://phabricator.kde.org/D20843