Update release description text in upload.py

[mbacchi]

Requested text to be used:

*This is not the released version of Brave. **Be careful** — things are unstable and might even be broken.*

These builds are an unpolished and unfinished early preview for the new version of Brave on the desktop. These builds show our work in progress and they aren't for the faint-of-heart. Features may be missing or broken in new and exciting ways; familiar functionality may have unfamiliar side-effects. These builds showcase the newest advances that we're bringing to your browser, but this is still a prototype, not a reliable daily driver. Try it out only if you're looking for a little extra spice and adventure in your browsing.

### Linux install instructions
http://brave-browser.readthedocs.io/en/latest/installing-brave.html#linux

### Windows
`BraveBrowserDevSetup.exe` will fetch and install the latest available version.

Until we have brave_installer.exe working without the need for adding cmd line
args it is unlisted in favor of the stub installer above to avoid confusion.