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

Tags: #kde_pim, #libkgapi

Differential Revision: https://phabricator.kde.org/D20843