Adds warning to manual-install doc.

doc/manual-install.md

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 # Server Installation for Jitsi Meet
 
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+:warning: **WARNING:** Manual installation is not recommended. We recommend following the [quick-install](https://github.com/jitsi/jitsi-meet/blob/master/doc/quick-install.md) document. The current document describes the steps that are needed to install a working deployment, but steps are easy to mess up, and the debian packages are more up-to-date, where this document sometimes is not updated to latest changes.
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 This describes configuring a server `jitsi.example.com` running Debian or a Debian Derivative. You will need to
 change references to that to match your host, and generate some passwords for
 `YOURSECRET1`, `YOURSECRET2` and `YOURSECRET3`.