Update linux.rst

Added explanation how to add the terminus powerline font on current ubunt-based distributions. (A config file prevents bitmap fonts from being imported otherwise).
1. This must be a note, not a separate point.
2. ReST does not use *just* indentation for the code blocks. I would not format the file name using code block though, better use <code>:file:\`/etc/…\`</code>.
3. It is better to also add a description of the contents of the file in case some distribution has this setting in another file (e.g. something like “If you do not have this file and font still does not work, check whether there are some files with `rejectfont` tag that disable non-scalable fonts or specifically Terminus”).