Document the upgrade procedure

[foxcpp]

<mfashby> Hello -- is there an upgrade script? or a note in the docs saying how to upgrade? I'd like to upgrade 0.1 -> 0.2 if possible.             
<mfashby> I could just wipe and reinstall at this point (heeding your 'not critical' notice) but it'd be nice to have a documented upgrade process. 
<mfashby> Having skimmed the installation script, it might be safe to just re-run that. Trying.                                                     
<mfashby> Yep that worked, no worries.                                                                                                              
<mfashby> Might be worth making a note of it in the docs