Update for beginners_doc

[meghanasharp]

Added in some additional tips that I discovered as a brand new user, while working through the beginners doc.
Edits include: how to open & edit bashrc or bash_profile; reactivating the environment after running bash; the need to install wget in order to download RGI glacier outlines

[dshean]

Looks great! Thanks @meghanasharp.

I'll clean up a few items that are specific to OS X, like the open .bashrc (one of my favorite things about OS X, but won't necessarily work for other systems). I'll add something about text editors like nano vim or emacs, though not great for beginners...