add information about data privacy to branchwater web site

[ctb]

from an e-mail conversation, author luiz:

---

I'm enjoying the branchwater metagenome query tool. However, are the submitted
queries stored or used within your servers? I want to submit a couple of genomes
that are not published yet, and I want to make sure they are only available
after our manuscript is published.

They are stored only in memory while the search is happening, they are never
stored on disk [0].

I do check the server access logs from time to time to have an idea of how many
unique visitors we have, but this is only data that the HTTP server logs
regularly (time, IP), and doesn't include the HTTP request content (where the
query data actually lives).

[ctb]

cc @luizirber, sorry