Explain what data is collected when opting in

[TheLastProject]

Currently, the "Open Dataset" just has an Opt In / Opt Out button, with no explanation of what data is collected.

After some discussion in #general, this seems to be the text that needs to be added:

Collected data includes:
- STT recordings and transcripts sent to Mozilla to train their Deepspeech model (possibly human-reviewed by Mozilla staff)
- Wake word recordings sent to Mycroft and reviewable by anyone (no information except the recording itself is shared)

This should include a link to the tagger when it becomes available again.

[chrisveilleux]

A great idea. Mycroft is currently revamping its Privacy Policy. Part of this process is to be more explicit about what we collect and what gets sent to third party service providers. This issue will be resolved in the next version of Selene. See #86