Now that we have the ability to rebuild 3rd party plugins, we should make it easier to let others contribute.

Docs that need updating:

• https://github.com/redhat-developer/codeready-workspaces/tree/master/dependencies/
• https://github.com/redhat-developer/codeready-workspaces/tree/master/dependencies/che-plugin-registry (new README re: how to build 3rd party plugins, using which job, and where they're published)

We should also add a section about building vsix files to this doc:

• https://github.com/redhat-developer/codeready-workspaces-productization/blob/master/devdoc/building/jenkins-maven-builds.adoc
• https://github.com/redhat-developer/codeready-workspaces-productization/tree/master/devdoc#table-of-contents (new TOC entry)