Add CONTRIBUTING info to root README and remove local CONTRIBUTING

The CONTRIBUTING.md file points to the contributing guide in the JBoss developer repository. When the scripts build the HTML files from Markdown, the link to the shared repository is replaced with a pointer to the HTML file, which renders poorly.

The local CONTRIBUTING.md file is not really needed, so it would be better to remove it and just add the pointer in the README file.

For details, see https://bugzilla.redhat.com/show_bug.cgi?id=1148848#c4