Created Enabling U2F authentication (markdown)

Enabling-U2F-authentication.md

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+To enable U2F authentication, you must be serving bitwarden_rs from an HTTPS domain with a valid certificate (Either using the included
+HTTPS options or with a reverse proxy). We recommend using a free certificate from Let's Encrypt.
+
+After that, you need to set the `DOMAIN` environment variable to the same address from where bitwarden_rs is being served:
+
+```sh
+docker run -d --name bitwarden \
+  -e DOMAIN=https://bw.domain.tld \
+  -v /bw-data/:/data/ \
+  -p 80:80 \
+  mprasil/bitwarden:latest
+```
+
+Note that the value has to include the `https://` and it may include a port at the end (in the format of `https://bw.domain.tld:port`) when not using `443`.