Give a warning in documentation about installing gems with umask 0077

[maticmeznar]

I spent most of yesterday figuring out my problems with Sensu saying it cannot find the gems I installed, etc. It turns out that because my umask is 0077, installed gems were only readable by root and not by sensu user. Installing gems with umask 0022 solves the problem. People have filled bug reports against rubygems in the past about this particular behavior, but they were closed as WONTFIX. I just think a warning in sensu-plugins documentation would be prudent to avoid wasting time.

[mattyjones]

@maticmeznar I will add this as a note and close it then.

Thanks!