Update links in README.md

Summary:

• Change the HIG link to hig.kde.org
• Changed the bugtracker link to include the icons component
• Changed the contact links to include all VDG chat rooms
• Slightly changed the wording

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: alex-l, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28923