Extend docs

[wpoely86]

Add error:

$ ml load cluster
Lmod has detected the following error:  No default cluster can be loaded. You need to choose one: 'ml spider cluster' gives an overview 
While processing the following module(s):
    Module fullname          Module Filename
    ---------------          ---------------
    cluster/.defaultcluster  /etc/modulefiles/vsc/cluster/.defaultcluster.lua

Which means that /etc/bashrc was not sourced.

Add to part about collections:

Collections make it very easy to work with your own compiled software. First load all the modules that you need to compile your software. Then build it and test it. If everything works fine, save the current set of loaded modules to a collection. In every job script that uses your software, you simple have to restore the collection. No more meddling with lots of module loads.