Add release notes about mountSources.

[metlos]

What does this PR do?

This PR adds release notes about the mountSources attribute in che-plugin.yaml.

What issues does this PR fix or reference?

eclipse-che/che#12510

[skabashnyuk]

CC @dneary. @slemeur @rkratky can you take a look too?

[slemeur]

Nice work!
@dneary make sure that this is callout as a feature/capability that is dedicated to Che 7.

[dneary]

Is this for 6.18 or 6.19? Just checking when the change landed.

[skabashnyuk]

6.18

[dneary]

Thanks - and can I have an example of how plug-ins worked when they did not have access to the source code? Did they have to request a text buffer from somewhere, or something? I'm looking for an illustrative example of how this enables something new for plug-in writer.

[metlos]

@dneary - there was the possibility to manually define a volume mount that would make the sources available to the plugin. mountSources makes that process simpler, but more importantly it makes sure that all plugins get the sources mounted on the same location, as explained in the release note.

[dneary]

I think the text needs some clarification, but I will propose a revision in a follow-on commit. Thank you @metlos!